Solitude for me. All the quests besides a Mind For Madness and Wolf Queen Awakened are boring, a huge amount of fetch quests, that are in the same town - too many lazy people there! Morthal has one of the most interesting quests in the game, has a resident alchemist there (who everyone refers to as very knowledgeable), Phinis Gestor's mentor who's the only one can cure your vampirism, visionary Jarl
The only downside in Morthal for me is the lack of a general goods merchant. Even Khajiit caravans don't stop there, so selling anything that's not a potion is problematic. The swamps, however are an extremely good place for poisoners. A vast amount of Deathbell, River Betty, Swamp Fungal Pod, Giant Lichen can be found there.
For me it's a tie between Windhelm and Winterhold. Windhelm, of all of the major cities is dull and in the crappiest condition. Doesn't really help that I dislike most of the NPCs in that city either. WInterhold, well because it's not even fit to be called a town, let alone a city. The only thing of interest there is the College. Whiterun is right behind these two purely for the sake of me visiting it way too many times.