Nightmare

The original variable-stat creature tied to a basic land type, and the template every later "lord of its land" beater has been measured against. Where most early creatures carried fixed numbers in the bottom right corner, this one outsourced its body to the battlefield: the more Swamps you assemble, the bigger it gets, with no upper bound except your manabase. That design idea (a flyer whose size scales with a resource you were already going to develop) was radical enough in the early game that the printing has survived across decades largely unchanged, and you can trace its DNA forward to Lhurgoyf, Multani, Maro-Sorcerer, and the land-count beaters that followed. Flying is what gives the scaling teeth: a ground-bound creature whose power equals your Swamps would stall against any chump blocker, but an evasive body that grows with your land drops turns every additional Swamp into both mana and damage on the same card. The cost is the discipline. Six mana buys a creature that dies to a single removal spell, and that is the rate Wizards has been willing to print on this effect: the ceiling is real, but you have to survive to the turn where the ceiling matters, and a six-drop that leaves nothing behind when it dies rarely earns that turn. What endures here is the idea, not the impact; this is the proof of concept whose descendants did the winning.