Mox Sapphire

The bluest of the original five, and the one whose name became shorthand for "the most broken card in Magic." The Power Nine framing tends to flatten what makes Mox Sapphire structurally distinct: zero-cost permanents that tap for colored mana are a category the game has spent thirty years trying to never reprint, because the math is unforgiving. A Mox is not a free spell; it is a free permanent that pays you back every turn. The first turn it sits in play, it has replaced itself in mana; every turn after, it is a free point of mana compressed into a card slot that cost nothing to deploy. Paired with blue's historical monopoly on card draw and counterspells, Sapphire was always going to be the worst offender of the cycle, and the restricted lists across Vintage and the bans across every other format have borne that out. The design lineage is just as telling: every subsequent attempt at the effect (Chrome Mox, Mox Diamond, Mox Opal, Mox Amber, Mox Tantalite) has carried a restriction, a cost, or a condition, because Wizards understood by 1994 that the unconditional version was a mistake they could not unmake. Sapphire is the platonic form of the problem, and the reason "Mox" reads as a warning label in every design doc since.