Although not "single card" per se, I'd like to touch on cards that haven't been properly exploited yet have a lot of potential in my eyes: the Alara Reborn Blade cycle, formed by:
First of all, they're all two mana creatures that can be played in two different color combinations: Bantblade in Selesnya / Azorius, Esperblade in Azorius / Dimir, Grixisblade in Dimir / Rakdos, Jundblade in Rakdos / Gruul and Nayablade in Gruul / Selesnya. As you notice, the guilds overlap in every each one, which makes playing up to two Blades in a single allied color pair possible, and up to three of them in a shard-colored deck if you want to push it that route.
The next point is that they all have relevant creature types: Bant is Human and Soldier, Esper is an Artifact, Grixis is a Zombie and a Warrior (which is getting more and more relevant in Rakdos colors courtesy of KTK block), Jund is a Goblin and Naya is an Elf.
And the third, final and most important point is that they are all great above the curve, 2 mana creatures with a built in combat keyword and Delver stats... if you jump through some hoops. Those hoops are what I'd like to discuss: you'll need a multicolored permanent card in order for them to be any good. That permanent can be a 3 CMC permanent in order to curve into it and swing and/or a 1 CMC, in order to curve from it already as a 3/2 (a must in the case of Jundblade). These are the permanents I think that are the most playable for each CMC:
Those are the most playables I found through a quick Gatherer search. The first idea I have is that a GW Hatebears / Goodstuff deck with Nayablade and Bantblade as the focus could be the best one. Hushblade / Sureblade + Shield of the Oversoul is simply devastating and a nightmare to deal with, and Hushblade + Armadillo Cloak is pretty strong as well. Hushblade + Scourge of the Nobilis could also happen but the other two auras are better IMO. Could act somewhat like a midrange Bogles build, less all-in and with more goodstuff. Couple that package with Guardian of the Guildpact and you have a steady steam of hard to deal with creatures.
Second idea is a UW evasive deck with Thistledown Duo, Esperblade, Bantblade, Steel of the Godhead and Shielding Plax, which would act pretty much as a regular tempo deck with evasion, card draw and filtering, and counters. Esperblade is particularly great with Edge of the Divinity, makin it a whopping 6/5 flyer hitting on turn 3 with counterspell backup and that can't die to Doom Blade or damage based removal spells.
Rakdos or Gruul with Jund Hackblade, the right Borderpost and Riot Spikes or Manaforge Cinder seems absolutely brutal, specially when curving into Giantbaiting in both builds or into Tattermunge Duo in Gruul. Fists of the Demigod, while a 2 CMC card, makes both Hackblade and Grimblade combat nightmares (especially the last one, although Wither is kinda redundant) that can't be killed with Bolt, Doom Blade, or Victim of Night in Grimblade's case. Clout of the Dominus + Grixis Grimblade provides an effect similar to Hushblade + Shield of the Oversoul, forgoing the evasion in favor of less mana investment and killing everything in it's path.
With that said, my ranking in viability is:
1º Naya Hushblade
2º Esper Stormblade
3º Grixis Grimblade (close call with Stormblade due to being able to wield Clout and boasting an immunity to both Doom Blade and Victim of Night)
4º Bant Sureblade
5º Jund Hackblade (not enough good 1-CMC enablers for this one, having haste also means this is on the short end of the stick).
Interesting analysis! I tried hard to make Rakdos-Blade work. It's fun, but certainly was never competitive. I think your tech with the Borderposts is really interesting, though. That could be a good start to the strategy. Dimir seems like the second most interesting to me. First Strike and Flying seem like excellent abilities for cards with only 2 toughness.
I often felt they were strong, but never got around to using them or brewing decks that would use them.
Of course Naya Hushblade would be strong when paired with some 3 cmc Auras, but to pull that off you can't trigger it's Shroud until after you cast the auras (which will happen if the aura resolves). This means you'll definitely have to use Sureblades and other creatures to get more out of your Auras.
As far as 1 cmc hybrid cards, you did leave out Boros Recruit which adds to options for Hackblade.
First of all, they're all two mana creatures that can be played in two different color combinations: Bantblade in Selesnya / Azorius, Esperblade in Azorius / Dimir, Grixisblade in Dimir / Rakdos, Jundblade in Rakdos / Gruul and Nayablade in Gruul / Selesnya. As you notice, the guilds overlap in every each one, which makes playing up to two Blades in a single allied color pair possible, and up to three of them in a shard-colored deck if you want to push it that route.
The next point is that they all have relevant creature types: Bant is Human and Soldier, Esper is an Artifact, Grixis is a Zombie and a Warrior (which is getting more and more relevant in Rakdos colors courtesy of KTK block), Jund is a Goblin and Naya is an Elf.
And the third, final and most important point is that they are all great above the curve, 2 mana creatures with a built in combat keyword and Delver stats... if you jump through some hoops. Those hoops are what I'd like to discuss: you'll need a multicolored permanent card in order for them to be any good. That permanent can be a 3 CMC permanent in order to curve into it and swing and/or a 1 CMC, in order to curve from it already as a 3/2 (a must in the case of Jundblade). These are the permanents I think that are the most playable for each CMC:
1 CMC permanents:
3 CMC permanents
Those are the most playables I found through a quick Gatherer search. The first idea I have is that a GW Hatebears / Goodstuff deck with Nayablade and Bantblade as the focus could be the best one. Hushblade / Sureblade + Shield of the Oversoul is simply devastating and a nightmare to deal with, and Hushblade + Armadillo Cloak is pretty strong as well. Hushblade + Scourge of the Nobilis could also happen but the other two auras are better IMO. Could act somewhat like a midrange Bogles build, less all-in and with more goodstuff. Couple that package with Guardian of the Guildpact and you have a steady steam of hard to deal with creatures.
Second idea is a UW evasive deck with Thistledown Duo, Esperblade, Bantblade, Steel of the Godhead and Shielding Plax, which would act pretty much as a regular tempo deck with evasion, card draw and filtering, and counters. Esperblade is particularly great with Edge of the Divinity, makin it a whopping 6/5 flyer hitting on turn 3 with counterspell backup and that can't die to Doom Blade or damage based removal spells.
Rakdos or Gruul with Jund Hackblade, the right Borderpost and Riot Spikes or Manaforge Cinder seems absolutely brutal, specially when curving into Giantbaiting in both builds or into Tattermunge Duo in Gruul. Fists of the Demigod, while a 2 CMC card, makes both Hackblade and Grimblade combat nightmares (especially the last one, although Wither is kinda redundant) that can't be killed with Bolt, Doom Blade, or Victim of Night in Grimblade's case. Clout of the Dominus + Grixis Grimblade provides an effect similar to Hushblade + Shield of the Oversoul, forgoing the evasion in favor of less mana investment and killing everything in it's path.
With that said, my ranking in viability is:
1º Naya Hushblade
2º Esper Stormblade
3º Grixis Grimblade (close call with Stormblade due to being able to wield Clout and boasting an immunity to both Doom Blade and Victim of Night)
4º Bant Sureblade
5º Jund Hackblade (not enough good 1-CMC enablers for this one, having haste also means this is on the short end of the stick).
Thoughts?
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Of course Naya Hushblade would be strong when paired with some 3 cmc Auras, but to pull that off you can't trigger it's Shroud until after you cast the auras (which will happen if the aura resolves). This means you'll definitely have to use Sureblades and other creatures to get more out of your Auras.
As far as 1 cmc hybrid cards, you did leave out Boros Recruit which adds to options for Hackblade.