Haven't got to the dark brotherhood yet, just about to finish the thieves guild. The Thieves guild is the best so far for me. I like the whole stealth aspect of breaking in and then leaving (hopefully) unnoticed.
Asmundr said:The Mages College.
The storyline was just epic. That is all.
The dungeons were nice but it was so predictable that the High elf was going to betray you slash try to kill you.