Chronomaton

A mana sink with no off-switch: every untap step it offers to convert a spare generic mana into a permanent +1/+1 counter, and it never stops asking. The body it starts on is irrelevant; what matters is the slope, because the growth is linear and the investment is open-ended, so the card pays out only in games that go long enough to feed it. That makes it a creature defined entirely by the resource it consumes rather than the threat it presents on arrival. A turn-three Chronomaton is a 1/1; a turn-ten Chronomaton, left alone, is whatever your leftover mana over six turns says it is. The artifact type matters more than the Golem subtype here: as a colorless one-drop, it drops into any shell looking for a place to dump excess mana, and the counter it accumulates is the kind proliferation cares about, the wrinkle that turns a patient grind into something less patient. On its own it is fair to the point of being unexciting, the rate honest and the ceiling distant. The interest lives in the asymmetry between its trivial cost to deploy and the unbounded ceiling it carries, a tension early colorless-creature design returned to often: cheap to play, expensive to fully realize, and only worth the activation when the board has stalled and the mana has nowhere better to go.