I'm going to go with Snorlax. Here's why: we're looking for the most dominant Pokemon in history, not just the most consistently useful. And while Pokemon in later gens certainly help shape their respective metagames, has any Pokemon ever hit mandatory status for usage? There's two that I can think of: Tyranitar and Snorlax. Tyranitar is arguably mandatory in Gen 3, but there's plenty of options there, I've seen a few non-Ttar teams floating around. Snorlax, on the other hand, depending on who you talk to, is mandatory in up to two generations. Unarguably in GSC, and arguably in RBY. The thing about Snorlax is that it pretty much single-handedly defines offense in both RBY and GSC with its attacking power on the Physical side (letting it bypass the difficulties Special Attackers face), incredible movepool, and of course the lastability that other physically offensive mons lack which occasionally makes them liabilities. True, Snorlax peters out in effectiveness starting in later ADV thanks to the near-ubiquity of Sand Stream and Leech Seed (it's no longer mandatory), but in ADV Snorlax is still a big threat that fills a niche nothing else can quite provide (Physically offensive all-purpose Special tank). Curse sets are also scary in the endgame and people must prepare for them. And sure, in DPP Snorlax might as well be a no-show with power being cranked up to the point where even the bulkiest things can't take the hits they once could, but who's keeping an all-time great from his position as such just because he plays a few mediocre seasons in the twilight of his career? (A more apt analogy might be keeping Michael Jordan out of the hall of fame just because he appears to be a mediocre owner, a player in a totally different "game"). That makes no more sense than omitting the youngster from the All-Star game.