You see, this is a pretty hard thing to answer, because of the nature of a good antagonist.
Often times, the best antagonists are deep, complicated characters. Two great examples come out of Bioware's camp; Teyrn Loghian and Saren Arterius. I dare anybody try to say they're truly evil people after having played the game trying to learn more about them rather than just saying "I hate you! Pew pew pew!" every time you encounter them. There aren't a whole lot of well developed antagonists that you could say are definitely evil, just plain simple rotten bad guys.
So you're usually left with enemies that just made your completion of the game a living hell because it was a tough boss battle or something like that. You don't hate the guys like Frank Fontaine as much as that one fucking vorcha in the Archangel mission in ME2 that had the flamethrower (you know which one I'm talking about). You hate the minor enemy for the amount of frustration he has caused more than a character who has done something extremely heinous, like shooting a child or such.
