RDj is the movie Stark/Iron Man that's for sure. But, I really don't remember the comic book Stark being quite having the smart-alecky, line a minute vibe that the movie Iron Man had.
So while Robery Downey Jr./Jon Favreau did a great job with Stark/Iron Man - it's not real faithful nor was he 'best cast' as Iron Man. They basically created a new version - a version that worked very, very well.
Same with Chris Reeve as Superman. I'll argue his Superman was a pale version of the comic book version of Superman. I'll acknowledge that many people look at him the embodiment of Superman, but I personally don't see him as a great Superman... but, that could be as much the writer/director's fault as his.
I do think Brandon Lee was amazing as The Crow.
Jeffery Dean Morgan as The Comedian worked pretty well for me.
Chris Hemsworth's Thor. Amazing casting choice there... the man has a charm that just works so well as Thor on screen. Probably the most spot, perfect casting as a comic book god.