Phyrexian Vatmother

The body is the whole point: a 4/5 that hits for poison is already an above-rate infect clock, big enough to survive most of the small creatures that would block or trade with it, and that durability is rare in a mechanic that usually lives on fragile one- and two-drops. The upkeep clause is what complicates it. Infect is a race you are supposed to win or lose; this card is the only thing handing poison counters to its own controller, turning the ten-counter loss condition into a clock that ticks against you as well as the opponent. You are not racing the opponent's life total or even just their poison; you are racing your own slowly filling poison count, which caps how long the Vatmother can stay on the board before it kills you. That self-inflicted pressure is the cost that pays for the oversized body. It reframes the card from a value piece into an accelerant: it forces commitment, rewards a deck already planning to end the game fast, and punishes any build that wants to grind. A creature that helps you lose is a strange thing to want, and the trade only makes sense in a strategy where the poison threshold is a finish line you are sprinting toward, not a wall you are trying to stay away from.