The awesome thing about Robin Williams is that it's hard to compare his movies because of the breadth of characters he played. His variety is greater than about anyone else I can think of - I mean, he played Mrs. Doubtfire and John Keating. Peter Pan and Sean Maguire (Good Will Hunting). He voiced the genie in Aladdin. Such variety there. But he was almost always a character you felt sympathy for.
Good Will Hunting is definitely my favorite. I get chill bumps every time I see it. Dead Poets Society and Awakenings are both right behind.
Good Morning, Vietnam was awesome but in a totally different way. He was outstanding in Aladdin and Mrs. Doubtfire.