Tough choice as I thought every DLC for Skyrim was flushed out and better than the main quest, but I would have to go with Dragonborn. I loved the perks you got from finding the black books. Solstheim was very intresting to explore. They also gave some love to mages through the Telvanni wizard questlines. WEREBEARS! RIEKLINGS! and SILT STRIDERS!. Also the daedric realm of hermaeus mora was intresting. The only disappointment was the uncontrollable dragons, but that was it. Dawnguard was a very close competitor though.
Dawnguard by far. Fighting vampires, hunting down Elder Scrolls, travelling to the Soul Cairn and fighting an undead dragon, and then the whole snow elf plotline, all of it is just so perfect. (also Serana is my favourite follower)
That's a tough one, I really like them both. I really like Dawnguard for also adding new locations to the vanilla portion of the game, like Dwemer ruins and vampire lairs. I guess I can't say Dragonborn drew me in as much, as I never 100% completed the main quest. I still want to finish it in a future play-through though.
Dragonborn was my fav, loved the story, and especially the Telvanni Wizards
Ironically, the quest that's stood the test of time in my memory is that one big Dawnguard sidequest where you help a woman's ghost find the aetherium forge. Not sure exactly why, but that's the questline that appealed to me most, more than any of the main quests.