Was he the best actor ever?

Who can say...

One of....

No, I don't think he does. His greatest performances were stellar, to be sure. But he also had lots of bad performances that he just phoned it, to collect a paycheck. And especially later in his career, he could be extremely flaky and unprofessional.

Even his very best work, great though it is, does not totally eclipse the work of other great actors like Spencer Tracy, Laurence Olivier, George C. Scott, Sidney Poitier, Daniel Day Lewis, et al.

I honestly admire actors like Spencer Tracy and Laurence Olivier more, as not only could they deliver equally magnificent performances, but they were also more professional and hardworking, more considerate of their fellow actors, and less prone to self-indulgent, flaky, or just plain weird behavior.

It's all subjective, and every actor is a different type, with different attributes to offer. There is no "best ever". Brando was often very, very good, and deserves accolades; however, saying he was the best actor ever is overrating him.