I would have to go with the thieves guild. That questline had an awesome story line and cool/likeable characters, a decent bad guy and above all the ability to actually rebuild the guild(this is what makes this questline better then the brotherhood).

I literally took my time with the thieves guild, made a character that did nothing but the thieves guild(and side quests) and managed to finish the questline at level 44 (i also didnt fast travel -unless by carriage morrowind style- and was on master difficulty so you level up way faster). By the time i finished the thieves guild, i had an entire town in the ragged flagon with one of every kind of merchant a fence with 4000 gold and badass items. Also in almost every other town there was another fence with another 4000 gold on them and the kahjiit caravan was also a fence with another 4000 gold.

When you finished the thieves guild and do all the extra [censored] as you go, you actually feel like you earned your spot as guild leader above all the other (older more veteran) members of the guild because they were going down the [censored]ter til you showed up and did a [censored]load of stealing and [censored] to rebuild the guild back to its former glory.

Guilds were all relatively poor but each had at least one good quest. Going with Winterhold because of Labyrinthian. However, the real gems are some of the daedric quests.

I think it's the Thieves Guild. It has one of the better storylines , the random quests are varied, so they don't get old too fast, and the expansions are worth doing them.