Butcher Ghoul

A 1/1 that has to die twice is exactly the kind of body sacrifice decks were waiting for. Undying turns the first death into a free upgrade: the Ghoul comes back as a 2/2, which means the value isn't in keeping it alive but in killing it on your own terms. That single +1/+1 counter is the whole transaction, and it's also the off-switch: once the counter is on, the next death is permanent, so the recursion is strictly a one-time loop. The design rewards a deck built to convert death triggers, drain effects, and sacrifice outlets into a second body and a second trigger before the counter shuts the door. It pairs naturally with anything that wants creatures entering and leaving the battlefield repeatedly, since the first round of that cycle is free. Cheap enough to function as expendable fodder and durable enough to demand two removal spells from an opponent who can't answer it with a single point of damage, it sits in the narrow band where a small, unassuming creature does real grindy work off the strength of one trigger. The counter is what keeps it from being an infinite engine on its own; you supply the second sacrifice, and the Ghoul supplies the value once.