In my opinion, The pokemon that teams up the best with Ash is Bulbasaur. In my opinion, it does beat out Pikachu. Remember the episode where they battle in that championship against that skiploom? They were such a great team. Even when they battled against Hitmontop in that Dojo episode. Also, against the Rhyhorn in the episode where it was supposed to evolve. I could comfortabley picture Bulbasaur being Ash's first and most important Pokemon, if that's how the series started. It would seem just as, if not more right than with Pikachu. There was just that spunk about it that was great. It should have stayed with Ash throughout the rest of Johto, and through Hoenn. At least I think so. It would have been better if it could have seen everything in Hoenn with Ash.
I basically think the same thing about 70% of Ash's pokemon, (Only that they should see everything with him, not that they could have been Ash's first pokemon or anything else that I just said.) just because he basically abandons them after each journey he goes into, but Bulbasaur is the best. Hands down.