Berserk

Pay a single green mana to double a creature's offensive output, with the cost paid in the creature's life: that is the deal, and three decades of green pump spells have been priced against it without ever matching it. The +X/+0 scales off the attacker's current power, so the spell rewards stacking it on top of other pumps and on creatures whose power was never meant to be doubled (a hexproof fattie, a Phyrexian Dreadnought, a trampling commander), while the trample it has granted since its first printing means none of that doubled damage gets wasted against a single chump. The destroy clause is what keeps the rate honest at one mana: this is not a combat trick you cast on a token to push two extra damage, it is a one-shot weapon that asks you to win on the swing or accept the loss. Giant Growth, Vines of Vastwood, and the various +X/+X-and-trample variants all came after it and all play it safer, which is why every reprint conversation around the card has been a power-level conversation rather than a flavor one. The lineage it spawned (the load-bearing finisher in every infect, Dreadnought, and one-shot-combat shell that wanted a green splash) is downstream of one decision in 1993: let a creature swing for double, and make it pay with its life.