Spelljack

Counterspells take something off the table; this one takes it and hands it to you instead. The triple-blue commitment is steep, and at six mana you are rarely the one with tempo to spare, but the payoff is theft rather than denial: the spell that would have ended you becomes yours to cast, free, whenever its timing rules allow. The exile clause is what makes the larceny stick (no graveyard recursion for the original owner, no flashback shenanigans, just a card sitting in exile with your name on it), and the "for as long as it remains exiled" wording leaves you a standing option rather than a one-shot, so you can sit on a countered Wrath of God or a fatty until the moment it hurts most. The X-becomes-0 rider is the honest tax: steal an X spell and you get the husk, not the bomb. Where a clean counter is symmetrical (their card and your card both leave the game's economy), Spelljack swings card advantage hard if it connects, which is exactly why it asks for six mana and three of one color. It is the maximalist end of the counterspell design lineage, less interested in efficiency than in the fantasy of turning an opponent's best turn into your best turn.