Eureka

Commander is where Eureka does its best work — four players means four hands dumping onto the battlefield at once, and the player who built around the effect (Progenitus, Blightsteel Colossus, a suite of planeswalkers) wins that exchange almost every time. The symmetry is a real cost, so it rewards tight deckbuilding: you want your hand full of permanents that immediately win or lock the game, not incremental value pieces your opponents can match. In Legacy, Eureka sees play in Show and Tell-adjacent combo shells where it cheats out an Emrakul or Griselbrand as early as turn one off fast mana, functioning as a second copy of Show and Tell with more variance. Vintage is legal but Show and Tell is cheaper and faster, making Eureka a fringe inclusion at best. Outside those three formats, it simply isn't legal.