This arguably is one of the most difficult questions posed to me. Before I come to a decision, I figure it's best to filter it by region and then from that selecting my favorite. For Kanto, I've been a fan of Bulbasaur. Johto, hands down Totodile. Hoenn,have to go with Sceptile. Sinnoh... so many good choices to choose from but I'll go with Gliscor. Unova is easy with Snivy and Kalos... going to go with Pikachu(rest are too cliche picks).
From that list of Pikachu, Bulbasaur, Totodile, Sceptile, Gliscor and Snivy, I'd have to go with Snivy. Ash doesn't own too many Female Pokemon and Snivy had a unique personality(not to mention I really like its design and Evolutions).