Platinum Angel

The win condition flipped on its head: instead of telling you how to defeat your opponent, it declares that you simply will not lose and they simply will not win, for exactly as long as it stays on the battlefield. That clause turns the whole game into a referendum on a single permanent. The fragility is the entire game it creates: as long as the Angel sits there, your life total can be negative, your library can be empty, you can be holding zero cards against a board of twenty power, and the loss never resolves. The instant it leaves, every one of those death sentences catches up at once. So the card spawns two opposite playstyles. One protects it: counter the removal, give it hexproof, untap into a hand of shields, and let an otherwise-lost game grind on indefinitely. The other races to kill it before the opponent can stabilize behind it, because a creature that rewrites the rules of victory is only as durable as its body, which is to say not very. Targeted removal, edicts, and a single sweeper all dismantle the lock as cleanly as they would any other creature; the flying only narrows which blockers can trade with it in the rare game where combat matters at all. Seven generic mana puts it in any color and any deck, so it has lived in artifact ramp shells, in joke combos, and in genuinely competitive lock pieces alike. It demonstrates as cleanly as anything in the game that a single ability, with no protection attached, can be both unbeatable and trivially answered.