Tilonalli's Skinshifter

Tilonalli's Skinshifter is a Commander card through and through — the format's slower clock and creature-heavy tables give you repeated triggers and worthwhile targets to copy every turn. In aggressive creature decks, especially ones running Voltron or combat-centric strategies, the Skinshifter adds a second attacker at the power level of your best threat for two mana, which scales obscenely well. In 60-card formats like Modern or Pioneer it's too slow and too conditional — combat-focused two-drops have a much higher bar, and losing the copy at end of combat is brutal when games end on turn four. Legacy and Vintage have no interest in it for the same reason.