This is a fun topic! My mom recently said to me, "Your favorite destination is always whichever place you visited last," and she's not wrong...I almost always love the places I go. The two that were the biggest surprises though are Rome and Vienna. Rome because I went as a college student without many preconceived notions; I had always been obsessed with the idea of Paris and London, and just hadn't thought much about Rome, so I didn't have any real expectations for what it would be like or a burning desire to go there. I went with a friend and just instantly fell in love, with the food, the beautiful streets, the ancient history, the art, the gregarious people...all of it. Now it's one of my favorite places to return.
Vienna was similar. It just wasn't particularly high on my list, but an opportunity arose to do a long weekend there last fall, and from my first sip of coffee in the Cafe Braunerhof I felt at home. In hindsight I'm not sure why it WASN'T high on my list, because I am a huge classical music fan (Beethoven in particular), I love Habsburg history, and Vienna cafe culture is pretty much my platonic ideal of how to spend an afternoon. I ended up loving it so much that I went back a mere two weeks later for another weekend.
On a smaller scale, after being a little underwhelmed by Cambridge, I absolutely adored Oxford.