Please not: it is which Avenger movie he played best at NOT where you like him most.

I think its Avengers: Infinity War

I think his performance in Avengers 1 was just amazing. His character developed the most in Avengers. Before that movie we knew ironman as a rich guy with cool gadgets wanting to make the earth a better place. But in Avengers he came out to be a Superhero!

That would be Endgame,
RDJ, with his skills just made a perfect syn. Where Tony Stark and RDJ are almost the same person.
The way he Played the character in ED was Devine.

He regularly improvises the lines that Tony has to speak. For example, when Carol brought him back from space, at the end of his talk with Steve, he says a word “LIAR” (which was improvised). And that word, while seemingly not significant gives a much deeper understanding of what Tony felt at that time regarding CAP.

I think it is endgame because in all other movies he is happy ,angry,or some other emotion but not saddest part but as in endgame he did a amazing performance of a hero who is in pain

In Endgame the Ironman character was best form all movies . The last seen was the best seen ….