The Thieves Guild, I've played through the DB, Companions, College of Winterhold, The Imperial, and The Stormcloaks.

The Thieves Guild definitely came on top for me, solid story line, great characters, nice plot twists, and I loved becoming a Nightingale. Sweet armor by the way, http://www.youtube.com/watch?v=qcFVULEp4i8

Whoever wrote this story line, great job. You've gotten a Dark Brotherhood lover to love The Thieves Guild even more.

EDIT: The lighting is bad in that video, inside the hood there's a mask sort thing that blocks your face and all you can see is some glowing eyes, and on the back there's a full black cape.

I believed it was the best plot as well.