Marchesa, Dealer of Death

Marchesa, Dealer of Death is legal everywhere except Pauper, but competitive Constructed formats have no interest in a four-mana legendary with a slow upkeep trigger — she simply doesn't impact the board fast enough. Commander is unambiguously where she belongs: multiplayer upkeep triggers fire three times per round, meaning three opponents each turn are losing life, pitching cards, or handing over creatures before anything else happens. Oathbreaker is a functional home too, though the smaller life totals and faster pace make the upkeep drain less decisive than it is across a full four-player table.