I'm not a huge fan of the worlds that come with the expansion packs. After playing them for a few years they just don't engage me. I am still rather fond of Sunset Valley, as it is were I played most of my games.
I love custom worlds. Cladbridge on Stowe and Saaqartoq are my favorite when it comes to breaking away form the generic "town" feeling.
I'm excited to try out Red County, all though the cheapest houses need to be edited so my sims can actually afford them upon starting the game.
And I also love Arcadia, the challenge of dealing with such small lots is very enjoyable.
I also have a hard time finding cities, so I'll also bring up The Stoneshields / Fortress Rock, Evensdale County, Elmira City, Los Aniegos, and Setra.
There are so many more great worlds out there! I'd love more options for cities with row houses, especally ones with backyards so I can have pets. :3