I love how much Solitude has to offer in terms of a great blacksmith, wealthy general store, scenery is BEAUTIFUL, and I adore all the cobblestone and castles kind of theme.
However, I love how simple and sophisticated Whiterun is. It's got everything you need, it's right in the middle of Skyrim, it's got lots of quests to do and it's such an attractive city, full of diverse people. I just love how easy it is to navigate; there's not ten billion streets and homes, it's just a few main stretches and the layout is perfect.
It's a tie between those two :3