Peace of Mind

In Commander, Peace of Mind earns its slot in lifegain-matters decks and any shell that wants cheap enchantments or graveyard fodder — one mana for a repeatable effect is the kind of efficiency 100-card formats reward. In Legacy and Vintage it's too slow and too narrow; racing combo or interacting at instant speed costs more than one white mana and a card. Modern and Pioneer have access to it but almost never want it — the ceiling is too low against the format's threats. Oathbreaker is where niche lifegain synergies occasionally pull it back into consideration, especially if your signature spell rewards incidental life gain.