Even though I'm pretty bored with it, I think I'd go with the MQ. It has so many kinds of quests, not always the same kill this-and-that and bring me this-and-that. And, you don't even know what you're doing. The puzzle is just slowly going together around you all the time.
Then again I, too, like Thieves Guild, especially Bal Molagmer. That was pretty much what I wanted to do with my first character ever, so I was happy to know the Guild is not just a bunch of thugs but did just what I wanted to do.
Mages Guild also has pretty eclectic questline. It's cool what you can do with poor Ranis there.
And Trebonius is a persona unlike any other.
He'd make a great Telvanni!
Also, Bloodmoon is very epic. Still have to go with MQ though because of its sheer length, versatility and mysteriousness.