For me the biggest disappointment has been Thor 3 and 4. In Thor Ragnarok, they took several elements of World War Hulk, and used them. We didn't get a proper World War Hulk movie. We got the costume yeah, we got the planet and some of the side characters sure... but we never got, Hulk rampaging and beating the avengers down as the villain in a major movie. It would require Sentry, and they didn't add Sentry until Thunderbolts. So NOW they can do World War Hulk but they already blew half their load in Ragnarok so it's semi redundant now and it doesn't work with Hulk's character arc since they made him professor hulk off screen. It's fucked.
And Thor 4, which could have been the best Marvel movie. It had Christian Bale, one of the greatest comic book actors of our generation, playing Gorr the god-butcher. A villain on a scale so epic that it could have been truly amazing. It should have climaxed with a trio of time displaced Thor's from his 3 ages: young brash Thor, Current Thor, and Odin-force Thor from his old age in the future. 3 Thor's vs Christian Bale Gorr, should have been the coolest thing that has ever happened, or will ever happen.

Instead it was a Taika Watitties movie with a lot of subversive crude humor and ironic characters who don't take it seriously, and then a shoehorned love story and kids movie that don't fit the tone and take up precious time real estate from a better film.