Ehhh, don't know about that. Tony Stark is absolutely, indisputably, considered RDJ's pinnacle role. Same with everyone who played Spider-Man. Chris Evans as Cap. etc...happens fairly frequently.
You're talking about financial or pop culture success....I'm talking about the quality of movies and/or acting ability.
He could be talking about either, both, or something else. I'm saying if someone doesn't care about the MCU they're probably going to say RDJ was better in "Kiss Kiss Bang Bang". Hell, RDJ was just in "Oppenheimer", which is a more serious (for lack of a better word) movie than some comic book shyt. If he wins an Oscar for that then I'm not sure how someone could argue Ironman was the peak of his career.
Regardless, it's a moot point because he didn't specify what he meant.
Fred.