There are some groups of enemies that are just there in any game. You have to deal with them, but you don't really think about them after the fact. Then there are groups you actively dislike, either because they're obnoxious to fight or because of meta-game reasons. But then there are those other groups. The ones that are just pure fun to fight against, the ones you always take quests or missions for once they become available. They've got interesting story elements, they look visually distinct, and they're challenging but not annoying. It's at that point which you almost want to be allied with the villains in question, just out of sheer awesomeness. (EVE Online might have had this in mind when they first let you do precisely that.)
We ask you, our fine readers, what's your favorite antagonistic group? Are they a low-level enemy that you remember fondly, or a high-level group that you still fight against at the endgame? Have you liked them ever since you first encountered them, or did you slowly grow to like them as you were flung against them? What is it you like about them? It's silly, but we can't play a game for too long without forming attachments, so share yours with us this morning.