Here is Pauline Kael’s 1966 essay on Brando for The Atlantic:

Brando represented a reaction against the post-war mania for security. As a protagonist, the Brando of the early fifties had no code, only his instincts. He was a development from the gangster leader and the outlaw. He was antisocial because he knew society was crap; he was a hero to youth because he was strong enough not to take the crap. (In England it was thought that The Wild One would incite adolescents to violence.)

There was a sense of excitement, of danger in his presence, but perhaps his special appeal was in a kind of simple conceit, the conceit of tough kids. There was humor in it–swagger and arrogance that were vain and childish, and somehow seemed very American. He was explosively dangerous without being “serious” in the sense of having ideas. There was no theory, no cant in his leadership. He didn’t care about social position or a job or respectability, and because he didn’t care he was a big man; for what is less attractive, what makes a man smaller, than his worrying about his status? Brando represented a contemporary version of the free American.

Because he had no code, except an aesthetic one–a commitment to a style of life–he was easily betrayed by those he trusted. There he was, the new primitive, a Byronic Dead-End Kid, with his quality of vulnerability. His acting was so physical–so exploratory, tentative, wary–that we could sense with him, feel him pull back at the slightest hint of rebuff. We in the audience felt protective: we knew how lonely he must be in his assertiveness. Who even in hell wants to be an outsider? And he was no intellectual who could rationalize it, learn somehow to accept it, to live with it. He could only feel it, act it out, be “The Wild One”–and God knows how many kids felt, “That’s the story of my life.”

A few years after this essay was published, Kael praised Brando’s “comeback” in movies like “The Godfather” and, especially, “The Last Tango in Paris.”