Overall it has to be Snivy for me. Her personality, her moves, her character. But I'd like any Pokemon that Ash has, no matter what he catches in the future, I'll always like it, and as much as I would like to see her more, she isn't shoved down our throats like Tepig and Oshawott.
Like Palpitoad, hated that Ash caught it, but since he has it, I like it. I don't know why I have such strong feelings for any Pokemon that Ash gets, but no matter what Pokemon Ash would get, I'll still like it. Like Klinklang, hate that Pokemon, but I'd like Ash's Klinklang, and be upset if it was rarely used.
Even Pikachu, yes, I know I've said I hated Pikachu, but that's only because it's shoved down our throat, as the 2nd episode of the subway proved to me at least, Pikachu can be quite likeable when used properly. Otherwise it can die and never comeback. Just saying; Neglected Pokemon >>>>> Overused Pokemon.