Sudden Shock

Two damage on its own would barely register; what makes this a hard answer is the line of reminder text above it. Split second freezes the stack: once it is cast, the spell that wants to combo off, the creature that wants to sacrifice itself in response, the fetchland that wants to shuffle away the burn, none of it can happen. The damage resolves into a window nobody can act inside. That turns a mediocre rate into a reliable kill for a class of creatures built to dodge ordinary removal: the morbid sacrifice fodder that demands you let it die on its own terms, the regenerators that want to tap mana in response, the one-toughness engine pieces that protect themselves with an activated ability. Against all of them, Sudden Shock does not negotiate. Note the precise limit of the clause, though: split second locks out cast spells and non-mana activated abilities, not triggers, so a persist or undying body still comes back when it dies. What the card buys you is the silencing of every instant-speed defense an opponent could choose, not an exemption from the rules engine itself. The design cost is deliberate: split second is the most restrictive timing clause in the game, so it gets bolted onto an effect that would otherwise be unremarkable, because a freely-castable, unanswerable spell would warp every interaction it touched. Pay two mana, accept that two damage is all you get, and the opponent's reactive options simply do not exist for the length of the resolution.