In RBY? Nidoking, not only is he an early route mon, he is an early full evolution in general and if you know your RBY glitches, the nanosecond it learns Horn Drill, you're never losing. But even without glitches, this fuckhead has single handedly the BEST movepool in the entire game. You don't even need legendaries.
I haven't replayed FRLG so i don't know which mon is the best there but i'm still gambling on Nidoking because again, early catch and easily the one mon that will carry your ass alongside your starter. Kadabra is a pretty good mon too but you get Nidoking way earlier
Gen 2, it depends entirely on the version because the best known crutch is Ampharos but Kadabra with elemental punches is awesome (though only in the gameboy color versions cuz Phys/spec split in HGSS)
Gen 3: Blaziken and Swampert are the GOAT starters, but regardless of your starter, Gardevoir is the game's own Nidoking in gen 3 with an amazing movepool and Calm Mind Voir is just sweeping shit.
Gen 4: Starraptor, no debate. You may get garchomp later on but Starraptor shares the same shit as Nidoking but tendfold because it's faster, its meaner and its a bird with Close Combat
Gen 5: Exca-fucking-Drill, enough said. This mon and Darmanitan are Unova's best.
Gen 6: Literally anything lol, megas are busted as shit
Gen 7: For some wack reason, you get Salamence stupidly fucking early but it also depends because gen 7 had a load of pretty fucking good early catches and by this point, the Magnemite line is the GOAT of all GOATs
Gen 8: Same as gen 6 lol, especially Cinderace.
