There are other actors that have given performances better than Brando's
Bardem in No Country For Old Men is better than anything Brando has ever done.
Fassbender in Shame..
Pitt in The Tree of Life..
Rourke in The Wrestler..
Pacino in The Godfather Part II..
But I'd still probably call Brando the best, because he was basically responsible for changing the conventions of acting into an artform rather than entertainment. And he's also given MANY great performances, so the points add up from those. His best work isn't far behind any of those mentioned above though.