A Feebas. They're actually way better than Magikarp, if only in that they learn more than splash. You shouldn't have much trouble getting it to level up and get stronger. I would say avoid grass or electric types though, since water types are weak to those two. They're excellent against rock, ground and fire types, and can put up a decent fight against ice and steel types.

I think that one evolves through the use of an item, but you'll have to ask someone else about that, since I don't have one myself.