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.