His most underrated film...

was the Australian film Candy. I can't believe he's not acknowledged for that role more often. I mean, don't get me wrong, he was in a lot of good movies. But I think Candy was his most powerful film. If you're a fan of Heath Ledger, it's an absolute must see. But be warned: it's a super ultra depressing movie. Probably the most depressing movie I've ever seen. But I say that in a good way. It's a masterpiece.