Hearthfire, it adds a new level of rp to skyrim. You can adopt children and make a house. Not only adopt the children, but give them gifts, clothes, weapons, food, and new clothes. Also your house can have giant attack and skeever infestations, I just find this so ufn, its like playing the sims in skyrim.
I'm partially torn between Dawnguard and Dragonborn as both had several pros and cons.
Dawnguard was of course the first DLC and added a whole new storyline to the game beyond what it already had to offer. There was a few new locations to explore as well as many interesting new characters. The Dawnguard offered several new unique weapons and armors while the new Volhikar clan offered powerful vampiric rings. Overall it was a pretty amazing DLC but does fall short to me since it's available so early on and therefor becomes repetitive when playing lots of new characters (also that final boss...ugh).
Dragonborn was then the "last DLC" for Skyrim (unless they're hiding the true last DLC) and was a bit bigger than Dawnguard. Adding in a whole new island to explore taking us back to Morrowind to battle another Dragonborn, Miraak. New creatures, new dungeons, and even more interesting NPCs AND a free home in Raven Rock that requires no upgrades (best part for me ). However, the final battle seemed more lack-luster than the fight at the end of Dawnguard.