Just a small list I've compilled together. Remember that the choices are my opinion and not official. You would also notice that I have less and less to say as we go down the list.
5 Hugh Jackman as Wolverine
When Hugh Jackman was cast everyone was a little skeptical, he was a last minute replacement for Dougary Scott and to top that off, he had a career of singing and dancing. But when everyone witnessed the good job he did Hugh's career changed, for the better. Although I must admit I put him on the list due to his sheer dedication to the role.
4 Jackie Earle Haley as Rorschach
I think this is the choice that everyone agrees with. From the way he moves to his voice, everything about this performence is spot on and I for one still think he should have got recognition for the performence outside the comicbook circle.
3 Christopher Reeve as Superman
Not much to say apart from the fact that Christopher Reeve is superman. The look was sheer perfection. Plus he had dedication because he appeared in superman 4.
2 Patrick Stewart as Professor X
I think hands down everyone wanted Stewart cast in the role. Ever since seeing him in Star Trek people could envision him as the professor.
1 Robert Downey jr as Iron Man
There was much speculation involving the role of Tony Stark. Many names were thrown around but no one really mentioned Robert Downey jr until he was indeed cast. Then people realised he was perfect. many stood up and said " Holy crap he's perfect!". And it was indeed true. If Tony Stark did live in the real world it would be Robert Downey jr.
Honerable mentions:
Ron Perlman as helboy
J.k. Simmons as J Jonah Jameson
Heath Ledger as the Joker
Thomas Jane as the Punisher
Wesley Snipes as Blade