1. RDJ - Like easily by a mile. He just brings so much personality to Tony. The first time I watched the Avengers I don't think I even saw anything else. Whenever he was on screen he just owned it. In then in AoU he plays a character that is definitely still 100% Tony Stark but is obviously a Tony who has been through hell and back. Also, I know some people complain about his making jokes at serious moments, but to me RDJ manages to put in enough emotion that you can tell when he's just being quippy to when Tony is using it as a defense mechanism.
2. Ruffalo - I had Spader second and then I remembered that actually there is one other character that I loved enough to notice in A1 on first viewing. Which is amazing because TIH is physically painful for me to watch. Just so much bad writing and (please note I adore Norton in a lot of his roles) it's like Edward Norton wanted me as a viewer to suffer. Then Ruffalo comes along and gives this performance that makes you just love Banner. Not even the Hulk, just Banner himself.
3. Spader - Don't even care what people say about Ultron. Loved him, and a lot of that was what Spader put into it. He just does creepy childlike AI that is throwing a hissy fit because he thinks he knows better than you so well. He gives this robotic character this absolutely human performance that is so perfect for the part. From his first scene asking JARVIS all those questions, figuring out who he was and hi purpose, I just knew I would love him. Also, just some great casting because he does a great job at copying all these Stark-isms.
4. Evans - A lot of people said Cap was poorly written in the first Avengers. I disagree. I think Cap has been a solid character in all his movies and since Evans is a common denominator there, I'm going to give him the credit. He does a great job at making Cap the unfun guy at times that you still absolutely adore. Looks terrible in helmets and masks, but I'm not going to hold that against him.
5. Bettany - Okay, so my first thought was just to go with Hayley Atwell (or maybe Clark Gregg) but Bettany did a great job with some really heavy lines in AoU (and also Atwell only had like a five second scene). No one likes getting exposition dialogue, but he managed to pull it off nicely. Plus he had all taht time as JARVIS, a computer voice inside Tony's helmet that I somehow managed to fall in love enough with as a character to have this reaction:
Vision: I am not Ultron
Me: Yay!
Vision: I am not JARVIS either
Me: What? Noooooo! JARVIS!
