The best Pokemon in LeafGreen is probably Mewtwo; you can get it in Cerulean Cave. Go up north of Cerulean City and across Nugget Bridge with all of the Nugget Trainers on it. Then take a left and go down the patch of grass. This next step requires a Pokemon that knows Surf. (E.g. Blastoise, Gyarados, Lapras) Surf south the Nugget bridge and Cerulean City and turn left, then return to land. You will see Cerulean Cave, then go in it. However, if you haven't beaten the Elite 4, the Champion (Your rival), and retrieved the Ruby and Sapphire in the Sevii Islands for Celio, then there will be a guy there blocking any unworthy Trainers who try to venture inside the cave. After you completed all these rigorous tasks, go inside and work your way through to the very end where Mewtwo will be seated.
hope this helps
Edited by: Luigi45