To me, the weakest link in the MCU has always been RDJ. I've never liked him as Iron Man...
My exposure to Iron Man in the comics is admittedly limited, but my understanding is that the whole 'Billionaire Playboy' thing was always a ruse to protect his secret identity, similar to Bruce Wayne in the Batman books.
In the movies, I've always felt that RDJ took the whole 'smarmy' thing to the 9th, which I found incredibly annoying and distracting. In fact, if RDJ, Ben Affleck, and Ryan Reynolds never made another movie, I could happily live with it.
Watching Die Hard again for the umpteenth time this past Christmas, I still maintain that Alan Rickman in his prime would have made the ideal Tony Stark...