This is tough to pick.. We have a bunch of gems in the MCU... But I'm going with..
RDJ, yes! I believe this was the perfect casting. He is no longer RDJ, he is Tony Stark. RDJ took the role of Stark and he knocked it out the park. His personality shines through the character and he also brought many features of the character from the comic to the screen. He has the attitude, the look, the whole shabang. It's sometimes hard to translate some characters from paper to the screen and he actually nailed it.
Yup, nailed it!