I think the Dark Brotherhood Questline was terrific. I originally was more excited about the Thieve's Guild but I wanted to have some sneaky armour so I started the DB questline to get The Shrouded armour at the start and just continue with Thueve's guild with the Shrouded armour being all sneaky. But I'd find myself keep coming back to The DB more than I would with the Thieves. I was terribly excited to get my next quest to kill the emperor, or maybe kill a bride at her own wedding (I do agree though, that was sad considering she'd done nothing wrong and it was just for attention). You get some truly worth-it rewards from this questline especially if you're a sneaky kind of guy and if you don't want to fix the new sanctuary you get 20000 gold at the very end which is VERY helpful. I think that the quests were very well written, especially the one where you came back to the sanctuary to see it burning to the ground surrounded by imperial soldiers. I was actually genuinely shocked to find out that *SPOILERS* Astrid betrayed me. I give this questline an 8/10 for it is my favourite. There could have been a bit more variety with the radiant side contract missions but altogether I reckon this is the best questline in the game!