This is a choice between TDW, GOTG, and Ant-Man for me. While the IM sequels weren't great, they weren't bad (except for the villain in IM2...that was really bad). Perhaps because I really like Thor, a lot of the stuff that worked in TDW really made the movie enjoyable, but I recognize it for the poor movie it was, with poor pacing and editing, and the final battle was anti-climactic, and then having Loki back on the throne pissed me off. GOTG had some terrible characters, a romance without any chemistry, and it cemented Thanos as all bark and no bite, which is a terrible lead in to the Infinity Gauntlet storyline. Still, it had memorable scenes, and the final battle was something I liked.
I had to choose Ant-Man, because really, there is nothing that happened in the movie that I remember, except for a number of things I didn't like. Now it wasn't a poorly paced movie, but it just seemed like movie-by-the-numbers, and was completely bland for me. The excitement I was supposed to feel at establishing future movies and characters was not there (oh. yay. the...Wasp?), and the story itself didn't make me invested in the characters. It was simply yet another "father reconnects with his estranged family and redeems himself by becoming a work automaton for them" Hollywood plot. Really insulting to men. It tried to have some kind of distinct identity beyond that at least, but the whole voice-over thing just felt gimmicky to me. Worst movie by being completely average and derivative, and not getting me interested in watching anything further within that character's universe. Which in the end isn't too bad for the MCU, if this kind of mediocrity is the worst they've done so far.