"Disliked" almost feels like too strong of a word for Ultron (which is why he's at the bottom end of this list). He wasn't outright disliked by the majority of audiences so much as he disappointed them, simply because they saw the potential that wasn't realized. The one thing that fans did dislike, however, was the portrayal of him.
Ultron was the big bad of the second Avengers movie, aptly titled Avengers: Age of Ultron, and he is an artificial intelligence created to protect the world by Tony Stark and Bruce Banner. Unfortunately, it all goes wrong and the AI being becomes self-aware, quickly shutting out his creators, destroying Jarvis and doing a quick analysis of the internet, only to deem humanity unworthy of inhabiting the planet.
The portrayal was drastically different that the comic book version, as Ultron was much more ruthless in the source material. He didn't always stop for these moments of reflection like the MCU iteration did, and it might have been better if they adapted him more accurately. Yes, he could be cold at times, but he would follow it up with quips and justifications and it just felt like the writers wanted to have their cake and eat it at the same time. Ultron was everything all at once, and that made it hard to understand him.
He's probably a bit over-critiqued, which is something that is being recognized because fans are responding more positively to Age of Ultron as the years go by, but he just didn't hit the mark in the same way that Loki did in the previous Avengers movie. The potential was there, but it never quite came together as seamlessly as Marvel Studios hoped it would.