Dark Brotherhood, boring? You're crazy!
The DB quest line is by far the best written of all. It's the only one that's emotionally engaging (not in a Final Fantasy way, but still), has really interesting quests that aren't just quests where you have to go and kill victim A in location X, but they're rather more creative most of the times. I was really sad when I was through with it. But I guess I can play it again and again, because this guild is the only one that makes me put the words "I love my job!" into the mouth of my character!
Being good means getting better.