The BEST?
that is hard to say for sure because thare are many types advantages and disadvantages in the game like amp plains, treeko with energy ball is deadly. but in the steam dungeon with all the fire types, not so much. however i will give this advice, when you choose your partner its a good idea to choose one that has a type disadvantage to you, (so if your a chimchar, choose treeko or bulbasaur) it can cover most weaknesses, that and diversify your team, Ive got water types, bug types, grass types, poison types, even fighting types., in a nutshell cover all your bases, and you'll be fine.