Hangarback Walker

The genius is that it answers its own removal. The doubled X means the same number buys both the counters and the casting cost, so the body that resolves dies into a swarm of flying Thopters equal to its size. That structure makes it nearly impossible to interact with cleanly: targeted removal trades down, board wipes refund you, and a sacrifice outlet converts the body into evasive fodder on demand. Exile is the only real answer, and any deck without it is choosing between letting the Walker grow under its own tap ability or eating a fistful of Thopters on the way out. Being colorless and an artifact, it demands nothing from the manabase, which is the other half of why it spread so widely: a mana sink for the early game, a payoff that scales into the late game, and a creature that punishes the very removal it invites. The activated ability is the slow burn that keeps the threat live; each counter added at instant speed is another Thopter banked against the death trigger, so a Walker that never attacks is still accruing value as a growing board presence. The design's lasting lesson is how cleanly it inverts the usual logic of removal: against most creatures, killing it is the answer, and against this one, killing it is the point.