Personally, my favorite one is The Imperials because they can actually offer Skyrim: order, more trading, and peace (most of the times.)

Dawnguard ( if DLC's count), I think it is a well done faction for both sides ( though I have never finished the Volkihar side). I like the fact that you can extend the main line of the Quests by doing Radiant quests. Also that their are plenty of "side" quests and places that you can pause and do other things rather easily. I never feel quiet as pushed as in the other factions to get to the next quest in the line.

* I know some people don't like the radiant style quests, I happen to*

My favorite DLC faction was mainly the Dawnguard as well. I only sided with them because of the crossbow and Serina.

The Dawnguard. It probably has my favorite main faction quest, a cast of quirky but likeable NPCs, and radiant quest rewards that are worthwhile. If you count Serana as part of their faction, that's just a bonus since she's my favorite follower.