There is a game that is widely regarded as the first fantastical/scifi/post-apoc game, and that game is Metamorphosis Alpha.
Adventurer! This Metamorphosis Alpha Bundle from Goodman Games features the vintage 1976 First Edition of Metamorphosis Alpha, the tabletop roleplaying game created by James M. Ward of mutants and monstrosities on a colossal generation starship. In 2290 humanity launched its first colonization ship, the Warden. A radiation storm in deep space struck and crippled the gigantic starship. The ship's human crew died -- the few survivors mutated and formed primitive civilizations -- tribes struggled against a world they could not comprehend. Generations later, as a human, mutant, or robot on the Warden, you fight to survive, unaware even that your irradiated world is a spaceship. Can you uncover the secrets of the Warden and steer it back on course, or will you simply try to live another day?
Yep. Before Gamma World, before Giantlands, James Ward created Metamorphosis Alpha. The First Edition of MA is like a snapshot in time, a waypoint in gaming history, and something that every old-school gamer should read at some point.
For 9.95 the Metamorphosis Alpha Bundle snags you Metamorphosis Alpha 1E, 3 Metamorphosis Alpha sourcebooks, and Death Ziggurat & Warden Adventures.
For approximately 26 bucks, you can add in Doom on the Warden, Epsilon City, and 4 Warden adventures.