I have seen another question like this, where the answer said that, aside from mew, the Pokemon that could learn the most moves was gallade at 88. I am not entirely sure about this, but I don't see any reason to doubt this person's answer.
http://pokemondb.net/pokebase/1123/what-pokemon-can-learn-the-most-moves
Also, technically, Smeargle can use any move, but he can't learn them all.
