I guess as far as body of work I'd say Ledger. I'm not familiar with Renfro's work... but I only know Lee from the Crow, which was a good movie but I wouldn't say his performance was all that amazing. I only know James Dean from RWOAC.. which again was a great movie, but I can't really judge based off of anything else. I can only recall River's performance in Stand by Me... great movie, but I can't say he was otherworldly in that.
I listened to a Bret Easton Ellis podcast with Larry Clark (Director of Kids) and he was talking about how Renfro was so fucked up on drugs he had to basically kidnap him and wait for him to detox in his car so they could shoot Bully (Which is an underrated movie btw). iirc he had another little story about Jake Gyllenhaal being pissed at Renfro because he was getting all the roles that he was auditioning for ...
Based on body of work this is easy. Ledger. Phoenix and Dean did show some serious potential. Dean however only played in like 3 movies when he died though, that would have been like Ledger dying after 10 Things, Monsters Ball, Knights Tale. Waaay too early to tell. As someone once said, James Dean died at the right time to make him a legend, if he had lived he likely wouldn't have lived up to it.
Easily Heath Ledger but I can see River or James being considered as well. James Dean become an icon with just the few films he did where as Heath actually had several good performances which kind of steals away from what James and River did. Dark Knight a side I think Candy or Brokeback mountain was easily the best acting Heath did.
I know 2Pac is better known for his music but I thought he was really good in the few films he did, Juice being the obvious one.
Fair point about it being hard to judge for most of them. Another role to consider for River Phoenix is young Indiana Jones. Yes, it wasn't inherently challenging role and it was in a popular crowd pleasing movie, but if you focus on him, he actually did a great job not just playing a younger version of the character, but playing a younger version of Harrison Ford playing a younger version of the character. In terms of body of work, I agree that you pretty much have to go with Heath Ledger, but I have always thought he was overrated. I am not a fan of these uber-method actors who substitute pretend angst for acting talent/craft. Related: fuck Daniel Day Lewis.
What are you serious? Where do you rank Daniel Day-Lewis? Btw great point about River Phoenix acting as a young version of Harrison Ford playing a younger version of the character. That so true. Brilliant acting by River Phoenix.