I think there is no simple answer to that question. A great city has a vibe of it's own, something that makes it different and worthwhile to visit. I enjoy a strong cultural presence, local traditions, food and events.
Personally I like a city with history, monumental buildings and old style streets that give light to your imagination.
Cities that are build to adapt to the environment instead of replacing it, building along the banks of a river or adapting structures and roads to slopes or building around hills or forests.