Ledger came to prominence with Brokeback Mountain. He was brilliant in that movie. I found his scenes at the end very moving, I never thought I would have felt that way about a love story between two men, but Ledger made you care for his character.
I don't have to explain at how awesome he was in the Dark Knight. For me, when you discuss Ledger it's a case of what could have been. Could he have been the next Brando, who knows? And when you think of it that way it's just too hard to compare him with anybody.