This was a fun little question to
ponder over. It was nice to go over my decks and think about what the single best card was. I'm assuming we mean the best card that isn't a combo piece.
Brudiclad: Probably
Irenicus's Vile Duplication. Being able to have a non-legendary copy if a legendary creature is a game changer. I can have a bunch of brudiclads, or a bunch of
Krenkos that exponential increase. And to top it off, they also have flying (for some reason). Very good.
Edric: Of all the extra turn spells this deck uses,
Notorious Throng is the best. But the card I like to see every game is
Gaea's Cradle. It gives me more mana than I know what to do with.
Haldan +
Pako: For this deck its probably
Chandra's Ignition. It wins games.
Pako gets big, so it's nice to have a damage spell that scales with him. I also like
Rishkar's Expertise in this deck for the same reason.
Harbin: This deck is awful. But if I had to pick a single card it would be
Myrel, Shield of Argive. She runs away with the game.
Krenko: Definitely
Skirk Prospector. It's mana ramp and a combo enabler. I also really like it when I get
Goblin Sharpshooter or
Goblin Assassin to keep everyone's creatures down.
Licia:
Bolas's Citadel. The lifegain in this deck is pretty good, so being able to turn that into free spells is wonderful. The only issue is that this deck does not—for the most part—have a way to get the lands
out of the way. I also really like
Fire Covenant and
Hatred, as they are other way I can weaponize lifegain.
Mishra:
Possibility Storm of course.
Scarab God: This one has a three-way tie. Like WWolfe's Sidisi I also like
Mesmeric Orb. It gives me a lot of cards to work with so I always have a response. The second pick is
Training Grounds. Halving the cost of
The Scarab God's ability is very good. That gives me twice as many activations. Lastly,
Sphinx of the Second Sun. In this deck it effectively gives me an extra turn. Any one of these three cards is a game-changer.
Tinybones:
Waste Not. I like getting a little bit more from my discards.