I'm going to say Sunset Valley as well. While I like all the towns, Sunset Valley is about the easiest to work with. Riverview has a lag issue for most people, and while the fog in Twinbrook is cool, it makes the animation a bit choppy when I'm focused in an area where it's rolling heavily.
Plus, I'm rather partial to Sunset Valley. It's got a lot of old favorites from days gone by and is a rather attractive town filled with attractive sims compared to the other two. To me, Sunset Valley never gets old.