Civil War.
In Iron Man he creates the character and defines our vision of live action Tony Stark (it'll be a long time before someone else will be so strongly identified with the character) .....but then in 4 subsequent movies ( IM 2, IM 3, Avengers, Age of Ultron) doesn't do much different - yes, some of that has to fall on the writing, but really the character and performance start to feel phoned in.
And then along comes Civil War, when we actually see Tony vulnerable again ( sorry IM 3 didn't actually convince me ) but in CW we see Tony really pissed off, wracked by guilt and frustration... at the pinnacle is that scene where he watches the recording of his parents' murder....man, that's incredibly emotional, and RDJ is so understated, you can feel the rage and grief, but he doesn't go OTT - simply brilliant acting.
Yeah, CW takes Tony to a whole new level (because the all-knowing smartass gets a bit old after a while) sure he has some good quips, but there's a gravitas in this one that we haven't seen since he was hooked up to a car battery in a cave.
So CW, with Iron Man being a very close second.