I like a well rounded, realistic character, as I do with my protagonists to be fair. No 2-dimensional mustache-twirling "I'm just evil 'cos I'm evil" unless there's a real good reason behind it or it's hella fun (think Andrew Scott's Moriarty). I don't need them to think they're doing good by being in opposition to the protagonist but I want them to at least be a sufficiently well written character that I could believe that they'd have an opinion on the matter & ideally feel some little bit of conflict over it like a real person would.