10. Whiplash
We mentioned that there are two types of "disliked" responses that a character can receive. A passionate response to being a good villain is a good thing, while an intensely passionate disliked of the missed opportunity is a bad thing. Unfortunately for Whiplash, he doesn't generate a passionate response in either.
The character was so underdeveloped and overlooked in his one and only MCU appearance that, over 30 movies later, it is quite easy to forget that he exists. That is so fault of Mickey Rourke's, as the veteran performer turns in a committed performance in a bid to make Ivan Venko a compelling villain in Iron Man 2, but the movie simply had too much going on to give its central antagonist the focus that he needed.
It's a shame really, as the beginnings of a great story were there, but the film's script simply turned Whiplash into a weaponized, enhanced threat for Tony Stark to take down. We had already seen that in the first Iron Man movie with Obadiah Stane, and he was passably one-dimensional as a business tycoon hungry for power, but Whiplash had a pretty compelling motivation to be more. Instead, we got an overstacked film that placed too much focus on Justin Hammer's vendetta against Stark, Tony's own self-destructive tendencies, building the foundations of The Avengers, and War Machine's beginnings.
The early days of the MCU didn't have all of the intricacies of a sequel worked out that they do now, but Iron Man 2 is still widely regarded as one of the franchise's most underwhelming movies and Whiplash is unfortunately part of the reason why. The material was there, as was the performance from Rourke, but the script let it down.