Snowpoint City. I like snow and winter, and it seems to always be snowy in Snowpoint. If I decided to just barely wander off from Snowpoint city, I could run into an array of pretty cool Pokemon, and I could evolve an Eevee into a Glaceon.
All of the cities in Sinnoh were pretty routine, nothing stood out too much. Snowpoint is the only one that's quite unique where it snows constantly (unless you count Floaroma town's flower lust.)