id say mudkip. he can be a bit of a force to be reckoned with in-game, plus its good to have a water type thats actually resistant AND strong against electric types due to it's half-ground type plus it can really pwn half the gyms on the game.
If you'd like a challenge, choose Torchic.
If you'd like the game easy, choose Treeko.
If you'd like a normal game, choose Mudkip.
I chose Mudkip, and it was a very good choice!