The Dawnguard, but only by default.
I hate murdering NPCs, which makes the Dark Brotherhood and the Volkihar Vampires super awkward.
I could get over stealing things, but the way I go around setting innocent people up and helping criminals stay unimprisoned makes me hate the Thieves Guild, plus an afterlife serving in Nocturnal's temple then being part of the mystic force that helps thieves does not appeal.
The Companions pass my morality check, but they fail questwise as I don't always want to be a goddamn werewolf, but if you want to continue the questline you are going to be a goddamn werewolf so suck it up, princess. And if that wasn't bad enough, you are then forced to kill an entire fort of Silver Hands to 'celebrate your transformation', then you get roped into Aela's little vengeance crusade. Neither of those things sit well for a good roleplay, and I resent being unable to say no or dob her into Kodlak or at least say: "I think I might have figured out why the Silver Hand hate werewolves so much, Aela."
College of Winterhold doesn't force me to me a horrible person or into questionable choices, but come on, we've all seen how ridiculous it is that the newest member (who may have stopped using magic after the novice spell for the gatekeeper) gets to be the new Arch-Mage. They seriously overlooked all the professors and three students who had been here way longer for a heavily armoured board and sword expert?
So that leaves the Dawnguard because both sides of the Civil War are complete losers. The only quest I hate more than Season Unending are the Civil War quests. I was actually amazed Isran survived Dawnguard, in all honesty, I totally thought he was going to die heroically and pass on the duty of killing all vampires to me. At the end I was basically: "I'm not the new leader of the Dawnguard because the old one survived? Yippee! Best faction ever!"