I will admit that I wasn't sure if Thanos should be on this list. On one hand, he's the Marvel Cinematic Universe's biggest of big bads; the threat it spent nearly 11 years establishing as a force-to-be-reckoned with who ultimately succeeded because he wiped out half of the universe, breaking moviegoers' hearts all over the world. On the other, he was the perfect example of a major villain story told right, inspiring fear, and even respect for how well the MCU pulled it off.
In the end, it wouldn't have felt right not to include him. That's why he's directly in the middle of this list, because after Avengers: Infinity War, fans hated him. But they loved to hate him, longing to see their favorite heroes take the fight to him and ultimately save the universe. He prevented them from doing that in that particular movie, but that made the wait for the rematch in Avengers: Endgame all the more worth it.
Thanos made Marvel fans feel strong feelings, and that's a sign of good storytelling. For years, they were intrigued by him, but when he snapped the likes of Bucky Barnes, Peter Parker, and Wanda Maximoff out of existence, he was the subject of their ire and animosity. And the real winner here was Marvel, because they had cleverly designed a villain who made everyone actively hate him for what he had done. And that's what separates Thanos from almost every other character on this list.