Ring of the Lucii

Ring of the Lucii is legal across every major constructed format except Pauper, but Commander is the only place it has a real argument. In Commander, legendary creatures are structural — your commander is always one, so the ring's restriction is nearly always satisfied, and repeatable instant-speed exile of nonland permanents is the kind of effect that compounds over a long multiplayer game. In 60-card formats like Modern or Pioneer, the legendary requirement is a meaningful tax: you need a specific creature type in play before the ring does anything, and four-mana Equipment that requires setup is a liability against faster linear decks. Standard legality may change that calculus slightly if the Final Fantasy set's legendary density is high enough to support it, but for now Ring of the Lucii belongs firmly in the Command Zone.