I love Dawnguard with all my heart.
Vampires, crossbows, husky and troll companions, awesome atmosphere and fantastic areas to traverse.
Castle Volkihar, Darkfall Passage and especially The Forgotten Vale are major highlights for sure.
Touching the Sky still remains one of my favorite quests of all time, everything about that quest is pure perfection.
...I think I genuinely like Dawnguard more than Dragonborn, I kid you not.