after really thinking about it I think that out of starters Charizard is probably the strongest but out of all either arceus or deoxys. arceus's downfall is that he can only really be one type at a time and so is vulnerable to attacks even if he has good base sttats and deoxy's downfall is that it's too one sided depending on its form.
Smeargle.
It can learn every move except Chatter.
His stats are not good,but its moveset is huge (all moves can be learned by Sketch,except Chatter) which means that it can be used both offensively and defensively (and even as a Baton Passer).
I am not a competitive battler, so I have no idea which Pokemon is the most useful, but my Infernerape is one of the best Battle Tower fighters I have in my arsenal. My Uxie also makes a pretty good damage sponge: huge Defense and Sp. Def., and knows Yawn, Dream Eater, and Protect.
I think Eevee is the cutest, though.
mewtwo. Or mew. or arceus.
Excluding the legendaries , probably Garchomp.