Progenitus

Commander is where Progenitus belongs — the format has enough reanimation, cheat-into-play effects, and legendary tutors to make ten mana irrelevant. In Legacy and Vintage it sees fringe play in Show and Tell shells where the mana cost never matters, but it competes with Emrakul, the Aeons Torn for that slot and usually loses. Modern, Pioneer, and Standard offer no reliable way to cheat it out, so Progenitus sits in those formats as a theoretical card rather than a practical one. Oathbreaker can run it as a signature spell target in niche builds, but the ten-mana ceiling is brutal without the singleton density of tutors Commander provides.