I'd have to agree that he was a fine actor, perhaps a great one - but the greatest actor of his generation? He was a contemporary of Alec Guinness and Burt Lancaster, for God's sake! Spencer Tracy was from an earlier generation, admittedly, but was producing some of his best work in the earlier part of Brando's career.
I think it's true that he was a very important actor. Someone in the paper was making the point that actors like Robert De Niro and Edward Norton could not have had the careers they have now without Brando opening the way.