I've basically written that it wasn't Brando but the vehicles he was cast in that made his career so uneven. I don't believe that "British director" would make such a pronouncement today. I would have pronounced in the Seventies that Sir Laurence Olivier was the greatest living actor and it's not a matter of proving anything. It's all opinion which, of course, and cannot be proven.
There are many contenders for the greatest male film actor. Certainly Brandon would be on my list of the top twenty in spite of some of his dubious career choices which were made, remember, after the studio system had collapsed. But not at the top of the list. Every performance I've ever seen has a mirror of the actor's own personality and character. Even Hannibal Lector where Anthony Hopkin's own intellect, devilish wit and polish shows through despite the despicable nature of the character. I don't think any of us can know how much of a portrayal is the actor's own personality and character.
Finally, it's not a matter of underrating Brando but a matter of overrating Brando.
as to brando, just look at what actors did before him, then after him.
like twain with literature, there was before brando and after him, and that chasm is wide, very wide.
I'm also in the camp that thinks Brando was a great actor.
I especiallly liked his subtle performance as Marc Anthony in Julius Caesar.
As for the Godfather, I also liked him as "Don Corleone." If his performance was in any way "bizzare," I think it was in perfect keeping wtih the Don Corleone character -- which, after all, is a "bizzare" role.
What is the right way to play an aging mafia don? I think the quiet, resigned, and yet dark intensity he brought to it was on the money.