Through 17 films, I would only count one MCU film as a disappointment (though some are certainly better than others) and that’s Iron Man 3. The only thing that has kept me from loathing the film more than I do was the Marvel One-Shot (I really miss those) that they did “All Hail the King” in which they fixed the film’s biggest blunder: making The Mandarin, Iron Man’s only good villain, into a complete joke.
Aside from The Mandarin debacle, Iron Man 3’s main problem is that it butchers Warren Ellis’ classic Extremis storyline, doesn’t do anything to advance Tony’s character other than blow up all his stuff, which he starts rebuilding before the credits finish, and the film generally stands outside of the MCU’s storyline. That’s fine for a lot of films, but Iron Man has been the backbone of the MCU since he started it, and to waste his last solo film was unforgivable.
There are two things about IM3 that are good enough that the film ekes out a positive rating (post-All Hail the King): Tony & Harley and the plane rescue sequence. I honestly could’ve done with a whole film of just Tony and that kid hanging out in Tennessee, but the film’s best sequence is a mid-air rescue of 13 free falling passengers after Air Force One is compromised. It’s an amazing F/X sequence, a great use of JARVIS, and some very creative problem solving by Tony. Would that the rest of the film had lived up to it.