Lightning Bolt

The baseline. Every burn spell printed since Alpha has been measured against it, every cheap red removal answer priced relative to it, and every format conversation about how aggressive red can be starts here. Three damage for one mana hits almost every early creature, a wide swath of planeswalkers, and twenty face damage spread across a handful of turns, all at instant speed and all with no restriction attached. That total lack of a clause is the whole story: Wizards has spent two decades printing alternatives that each pay a tax for the same output. Searing Blood needs a death-trigger to finish the burn, Skewer the Critics needs spectacle to cost one, Play with Fire trades a damage for a scry, Lightning Strike costs an extra mana for the privilege of being printable in newer Standard sets. None has displaced it where it is legal, and that very dominance is what keeps it out of so many places. The card's power level is high enough that its presence reshapes what a format's first turns look like, so modern Standard simply declines to print it, and several other formats have chosen to keep it out entirely. It is the closest thing the game has to a fixed point: a card whose rate is so clean that every era of design negotiates around it rather than with it.