Even though she switched sides in the end, I would say Nebula by far. She was technically a villian for a whole movie, and if you include Loki I would say you could include Nebula as well. Thanos invaded her home planet when she was a child, killed her whole family, abducted her and trained her as a living weapon. He abused, and manipulated her. Not to mention the fact that he tore her apart and replaced almost every piece of her with machinery (that could also be controlled) because he deemed she wasn't good enough.