Roxi, Publicist to the Stars

Roxi, Publicist to the Stars is banned in Legacy, Vintage, and Oathbreaker, and isn't legal anywhere else outside Commander — that's a card that exists almost exclusively in the 100-card singleton world. The three restrictions it carries (legendary-only trigger, token limit tied to your legendary count, and the token being a specific named creature type) look punishing on paper, but Commander is a format defined by legendary creatures at every level of the stack, so the legendary-spell clause fires constantly. The singleton rule that would dilute the engine in other formats is irrelevant when your opponents' commanders are legendary permanents generating their own triggers, and the political pace of multiplayer gives Roxi time to snowball in ways 60-card formats never would.