13. Iron Man 2
The worst film in the MCU is a hodge-podge of elements bumping into each other. While almost every Marvel film has at least some trouble balancing the needs of the film vs. the needs of the MCU, Iron Man 2 is the most egregious. Characters come and go as they please. Black Widow, so a fundamental part of the universe today, doesn’t seem sure what she is doing there while Mickey Rourke just wants his bird. Just give him his bird, dude. (Rourke out Leto-ed Jared Leto in irrelevant method acting for a comic book film by visiting a Russian prison for this role. One wonders if the actual Rourke is still over there).
You can see the desire to go darker, to perhaps attempt the famous “Demon in the Bottle” storyline of Iron Man comics past. But whether corporate mandate or creative choice, the most we get is “Stark’s Arc reactor? Turns out it’s killing him LOL” and a couple bad hangovers. The early days of Marvel were glorious but every giant trips once in a while. It’s probably best they worked out the kinks early.