My answer tho is Loki, specifically for Thor 2. I don't think he was great in Avengers and he was solid in Thor 1. He wasn't a villain the rest of the time.
But Thor 2 Loki is like my top villain in the MCU, along with Thanos.
"Bordate is a pretty shady place, what with the gangs, casinos, evil corporations and water park." - FAHtastic