First I should clarify what I mean by "Significant". I don't mean the most popular. I don't mean the most expensive. What I mean by this is, the cards that are the most responsible for defining the format as is exits right now. Some of these may very well also be seen on such other lists, so don't be confused by that. The cards on this list are the ones actively making the most impact in the format right now by either holding powerful decks back, or making decks more popular and creating a need to be expecting those decks/cards.
This isn't going to be a top 10 going from least to most significant, as while some cards may seem to easily earn their place on such a ranking, others would be harder to define compared to the rest. And it would just be to highly debatable to make such claims.
A lot of these cards a Legacy veteran would easily expect to see on this list, so don't read this expecting 10 seemingly random cards along with detailed cases made for their claim at being on this list. There will be a few listed though that are more debatable, and may not be as expected.
Force of Will
This is a staple for Legacy. And to many is the card that defines the format the most. Having a "No Button" that's so useable for so many decks and situations is extremely valuable. Any player on the play that plans on casting something the first turn, against a unknown deck in this format, is/should be thinking about this card. A first turn FoW, or lack of one defines the remainder of that match most often.
This is currently being used in almost all the most popular decks. Save for Maverick and Dredge. And it has a home in many of the less popular but quality decks.
This card single-handedly keeps powerful Combo decks from either being viable at all, or placing well in tournaments more often.
Dual Lands (original)
While these make up 10 cards themselves, not lumping them together would make for a very bland list.
These cards like others on this list define the format, and have done so for a long time. In a way they're deceptively simple, and a novice may not understand their power. Without these, the clocks on the average deck would be slower, decks would be less consistent on reaching their goals, and there very well might be a higher number of mono color decks and less tri-color decks contending for viability.
Fetchlands
These, like Dual Lands are lumped together to make this list less bland.
The ability to fetch the format defining Dual Lands, basics when playing around Wasteland, or even tailor what mana you have access to based around your hand, permanents and graveyard, makes these auto included in most decks in the format.
Additionally they're commonly used in combination with Brainstorm and Sensei's Divining Top to help ensure increased odds of drawing the best cards for your situation or game plan.
To some extent their useful to thin your deck as well, removing 1 land from your deck there by increasing your odds of drawing more relevant cards.
You even may on occasion see these fetch a Dryad Arbor as a surprise blocker/attacker, among other tricks.
Wasteland
In a format so defined by Dual Lands, and one with access to so many powerful non-basics in general, this card keeps all of those in check. Without this, there would be almost no reason not to run Dual Lands in mono color decks, giving the player access to the type of cards he wouldn't have access to otherwise.
An unprepared player keeping a Dual Land in opening hand, or fetching one early on could very well wind up locked out of the game for a significant amount of time in which he can't recover from to be able to turn the tide in his favor.
Sensei's Divining Top
With bans placed on a lot of the best tutors that would otherwise be available, the combination of deck manipulation plus shuffle effects form the next best thing. This card in particular provides the manipulation part of that, and does so with re-useability! Additionally this is put to great use with cards like Counterbalance and Delver of Secrets, to create some very significant swings or maintain position in matches. It also helps keep a Miracle on deck for when you need it the most.
Brainstorm
A long time staple of the format, this is another form of deck manipulation to combine with shuffle effects to replace the missing tutors. But this goes beyond that and allows you to replace potentially the 2 most useless cards in your hand with more valuable ones. All at the cost of only a single Blue mana. And at instant speed! This can often be used to make the card feel like your playing 4 Ancestral Recalls!
This like SDT Combos well with Delver of Secrets, and to some extent Counterbalance. And with Miracles, it can get an already drawn and otherwise dead one out of your hand and on top of your deck where and when you want it.
Grisebrand
Finding its home in Reanimator and Sneak and Show decks, this guy has caused a need for decks to be tuned and tooled to be able to deal with this guy or his repercussions when he hits play. In a big tournament you need to expect this guy and these decks, or else you are going to get rolled over and probably issued your walking papers.
Stoneforge Mystic
This card is currently finding it's home in many decks. It's little without the quality selection of equipment it searches for from pre-planned toolboxes. Cards like Batterskull, Umezawa's Jitte, and various "Sword of"'s are what it often finds, can put into play at instant speed, and in the case of Batterskull, at significantly less then its normal cost to play. It creates advantage in almost every possible way you could want with this selection.
Because of all this, this guy often can't be ignored and becomes the target of removal before he can put what he searched for into play. But often, even then the damage may have already been done.
Cavern of Souls
This card has contributed to a resurgence of pure Agro, Agro-Combo, and Tribal decks. Along with that increase, pure Combo is becoming more viable due to it's generally favorable game against such decks. This has caused the format to be the most balanced, open, and unpredictable than it has in a long time.
Scavenging Ooze
The other 9 cards on this list were much easier to decide on. The 10th was not so easy. I landed on this guy because he's multi-faceted. He grows big for just a 2 mana creature, gains life, and picks apart popular graveyard plans. Tarmogoyf was a very popular beast in the format for a long time, and this guy has caused his presence in the format to start diminishing because for the same casing cost he often grows bigger while making the goyf smaller. The ooze also has more use in other situations due to that very same GY hate
Decks that use the graveyard as an asset are common in Legacy, from Dredge, Reanimator, or various decks that use Flashback through either natural means or Snapcaster Mage. Any such deck better be planning on dealing with or playing around the ooze. They have to play around GY hate normally as is. But this guy resides in the main deck a lot making him more important then the cards that come in during game 2.
Notable Mentions That Didn't Make the List:
Swords to Plowshares - Without this, there would still be Path to Exile.
Snapcaster Mage - Only as strong as what it's giving Flashback to. Otherwise it's a 2/1 for 2cc with flash.
Delver of Secrets - A very solid creature, but requires certain decks to be useful, and his absence from the format wouldn't change things THAT much.
Lion's Eye Diamond - Used in many of the formats best combo decks almost to the extent of being a 4 of Black Lotus. But while weakened without it, decks wouldn't be to much different.
Green's Sun Zenith - Used to great effect in several quality deck lists, but just shy of being top 10.
City of Traitors/Ancient Tomb - Very good at what their used for, but what they lead to matters more.
Final Thoughts:
The format is wide open right now. Some decks are still more popular then others, but the lists of viable decks that could be seen winning any given tournament right now is large and varied. The coming months should be interesting to see what may shine above all else.
I don't have much for predictions. Agro-Control decks have proven themselves enough in the past that in the end that's likely were things will wind up again. Any time a Agro or Combo deck starts shining above the rest, it gets hit with a ban. If anything were to go that route, Griselbrand is of course topping that list. But I just don't see that happening with the known decks available right now.
With all decks being so equal, other factors will start mattering more for popularity. Generally money cost. The cheaper/easier to build decks would make more sense as popular choices. In such a wide open format, pure fun could start making it's way into the selection process more. But as people define fun differently, who knows.
For now, keep an eye on Agro and Agro-Combo. It's their time in the spot light!
This isn't going to be a top 10 going from least to most significant, as while some cards may seem to easily earn their place on such a ranking, others would be harder to define compared to the rest. And it would just be to highly debatable to make such claims.
A lot of these cards a Legacy veteran would easily expect to see on this list, so don't read this expecting 10 seemingly random cards along with detailed cases made for their claim at being on this list. There will be a few listed though that are more debatable, and may not be as expected.
Force of Will
This is a staple for Legacy. And to many is the card that defines the format the most. Having a "No Button" that's so useable for so many decks and situations is extremely valuable. Any player on the play that plans on casting something the first turn, against a unknown deck in this format, is/should be thinking about this card. A first turn FoW, or lack of one defines the remainder of that match most often.
This is currently being used in almost all the most popular decks. Save for Maverick and Dredge. And it has a home in many of the less popular but quality decks.
This card single-handedly keeps powerful Combo decks from either being viable at all, or placing well in tournaments more often.
Dual Lands (original)
While these make up 10 cards themselves, not lumping them together would make for a very bland list.
These cards like others on this list define the format, and have done so for a long time. In a way they're deceptively simple, and a novice may not understand their power. Without these, the clocks on the average deck would be slower, decks would be less consistent on reaching their goals, and there very well might be a higher number of mono color decks and less tri-color decks contending for viability.
Fetchlands
These, like Dual Lands are lumped together to make this list less bland.
The ability to fetch the format defining Dual Lands, basics when playing around Wasteland, or even tailor what mana you have access to based around your hand, permanents and graveyard, makes these auto included in most decks in the format.
Additionally they're commonly used in combination with Brainstorm and Sensei's Divining Top to help ensure increased odds of drawing the best cards for your situation or game plan.
To some extent their useful to thin your deck as well, removing 1 land from your deck there by increasing your odds of drawing more relevant cards.
You even may on occasion see these fetch a Dryad Arbor as a surprise blocker/attacker, among other tricks.
Wasteland
In a format so defined by Dual Lands, and one with access to so many powerful non-basics in general, this card keeps all of those in check. Without this, there would be almost no reason not to run Dual Lands in mono color decks, giving the player access to the type of cards he wouldn't have access to otherwise.
An unprepared player keeping a Dual Land in opening hand, or fetching one early on could very well wind up locked out of the game for a significant amount of time in which he can't recover from to be able to turn the tide in his favor.
Sensei's Divining Top
With bans placed on a lot of the best tutors that would otherwise be available, the combination of deck manipulation plus shuffle effects form the next best thing. This card in particular provides the manipulation part of that, and does so with re-useability! Additionally this is put to great use with cards like Counterbalance and Delver of Secrets, to create some very significant swings or maintain position in matches. It also helps keep a Miracle on deck for when you need it the most.
Brainstorm
A long time staple of the format, this is another form of deck manipulation to combine with shuffle effects to replace the missing tutors. But this goes beyond that and allows you to replace potentially the 2 most useless cards in your hand with more valuable ones. All at the cost of only a single Blue mana. And at instant speed! This can often be used to make the card feel like your playing 4 Ancestral Recalls!
This like SDT Combos well with Delver of Secrets, and to some extent Counterbalance. And with Miracles, it can get an already drawn and otherwise dead one out of your hand and on top of your deck where and when you want it.
Grisebrand
Finding its home in Reanimator and Sneak and Show decks, this guy has caused a need for decks to be tuned and tooled to be able to deal with this guy or his repercussions when he hits play. In a big tournament you need to expect this guy and these decks, or else you are going to get rolled over and probably issued your walking papers.
Stoneforge Mystic
This card is currently finding it's home in many decks. It's little without the quality selection of equipment it searches for from pre-planned toolboxes. Cards like Batterskull, Umezawa's Jitte, and various "Sword of"'s are what it often finds, can put into play at instant speed, and in the case of Batterskull, at significantly less then its normal cost to play. It creates advantage in almost every possible way you could want with this selection.
Because of all this, this guy often can't be ignored and becomes the target of removal before he can put what he searched for into play. But often, even then the damage may have already been done.
Cavern of Souls
This card has contributed to a resurgence of pure Agro, Agro-Combo, and Tribal decks. Along with that increase, pure Combo is becoming more viable due to it's generally favorable game against such decks. This has caused the format to be the most balanced, open, and unpredictable than it has in a long time.
Scavenging Ooze
The other 9 cards on this list were much easier to decide on. The 10th was not so easy. I landed on this guy because he's multi-faceted. He grows big for just a 2 mana creature, gains life, and picks apart popular graveyard plans. Tarmogoyf was a very popular beast in the format for a long time, and this guy has caused his presence in the format to start diminishing because for the same casing cost he often grows bigger while making the goyf smaller. The ooze also has more use in other situations due to that very same GY hate
Decks that use the graveyard as an asset are common in Legacy, from Dredge, Reanimator, or various decks that use Flashback through either natural means or Snapcaster Mage. Any such deck better be planning on dealing with or playing around the ooze. They have to play around GY hate normally as is. But this guy resides in the main deck a lot making him more important then the cards that come in during game 2.
Notable Mentions That Didn't Make the List:
Swords to Plowshares - Without this, there would still be Path to Exile.
Snapcaster Mage - Only as strong as what it's giving Flashback to. Otherwise it's a 2/1 for 2cc with flash.
Delver of Secrets - A very solid creature, but requires certain decks to be useful, and his absence from the format wouldn't change things THAT much.
Lion's Eye Diamond - Used in many of the formats best combo decks almost to the extent of being a 4 of Black Lotus. But while weakened without it, decks wouldn't be to much different.
Green's Sun Zenith - Used to great effect in several quality deck lists, but just shy of being top 10.
City of Traitors/Ancient Tomb - Very good at what their used for, but what they lead to matters more.
Final Thoughts:
The format is wide open right now. Some decks are still more popular then others, but the lists of viable decks that could be seen winning any given tournament right now is large and varied. The coming months should be interesting to see what may shine above all else.
I don't have much for predictions. Agro-Control decks have proven themselves enough in the past that in the end that's likely were things will wind up again. Any time a Agro or Combo deck starts shining above the rest, it gets hit with a ban. If anything were to go that route, Griselbrand is of course topping that list. But I just don't see that happening with the known decks available right now.
With all decks being so equal, other factors will start mattering more for popularity. Generally money cost. The cheaper/easier to build decks would make more sense as popular choices. In such a wide open format, pure fun could start making it's way into the selection process more. But as people define fun differently, who knows.
For now, keep an eye on Agro and Agro-Combo. It's their time in the spot light!