I think he poured his heart out in East of Eden which the role called for and which established his reputation. He played a variation on the same role in Rebel Without a Cause, which was somewhat out of place in the story - a vastly inferior period teen drama - but made him a mega-star and cemented the image most people have of him as an icon. He moved on to play yet another variant on the previous two roles, which saw him stretch only marginally in Giant, a very weak Rock Hudson/Elizabeth Taylor star vehicle.
The rest of his resume is blink-and-you'll-miss-him bit parts and TV performances, hardly an impressive body of work. His reputation as an actor rests more on the belief that he was gone before he reached his potential than the actual quality of the work he left behind. His contemporary Marlon Brando was a far more impressive actor but had the great misfortune of having to answer for a decades long run of bad career choices (with a couple of brilliant exceptions) that sullied his stature.
