Apologies, I suddenly got swamped with RL stuff and can't do the full written breakdown of each card. Pretty great ideas all around, though my main critique for many cards involve their mana value or activation cost one way or the other.

Spoiler: Runner-Up
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+5 Vorpal Bunny (Dimir Skinwalker)
So Dimir got a new Evil Twin, except it's also a 2-for-1 combat trick? I love it.

Spoiler: Winner
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mythmonster2 (Gideon, Guardian in Death)
I'm surprised we still haven't gotten a Gideon with this specific replacement ability, especially with Gideon's Sacrifice and Gideon's Intervention being a thing.