Snapcaster Mage

A 2/1 with flash is a body almost nobody pays for, which is the whole trick: the entry trigger is the spell, and the creature is a delivery mechanism that happens to attack. Flash means the trigger goes on the stack during your opponent's turn, so a single card answers their threat (rebuy a counterspell, a removal spell, a bounce) and then sticks around to chip in. The design genius is that it scales precisely with the deck around it: in a vacuum it does nothing, but the flashback cost it grants equals the spell's printed mana cost, so it never cheapens the rebuy and never breaks symmetry, yet it turns every cheap interactive spell already in your graveyard into a flexible toolbox accessed at instant speed. That tension between a fragile body and an open-ended effect is why it reshaped how blue tempo and control decks were built; the card became the reason to play low-mana-value instants, and the reason to leave them in the yard rather than reach for the cheapest possible answer in hand. It also rewrote the math on card advantage: every instant or sorcery in the graveyard is now a half-card waiting to be cashed, and the Wizard reading its own controller's discard pile is the closest blue has come to a recursion engine that asks nothing of the cards it reuses except that they were good to begin with.