If by "rubbish" you mean his weakest or least effective pokemon, I would have to go with Totodile. Cyndaquil is still a first stage, and Chikorita, at her first stage was not the strongest, despite its never say die spirit, neither was Cyndaquil, but at least they had some victories. Totodile had few, if any, victories at all except for the Team Rocket battles. All it did was dance and it was hardly serious about battle, even when the time came to fight and that cost him some matches. Every other pokemon of Ash's has at least some personality, and is willing to focus on a task when the time comes, regardless of whether it is battle or not. I will readily admit, though, that I am biased here. I like strong pokemon who are serious about what they're doing regardless of their evolutionary level. That's why I like Ash's Squirtle, Bulbasaur, and Corphish, even though they are still at their first evolutionary level.