I think I answered on this before but here goes anyway. I love Sunset Valley, as I seem to play there the most, but I also love Twinbrook. Those two worlds just have the most for my gameplay. If only they had supplied a train for those tracks in Twinbrook then I would be in heaven!
For Sims 3 store worlds, I adore Roaring Heights, Lucky Palms, and Lunar Lakes. They are so pretty.
I need to play in the other store worlds more - like Aurora Skies and Dragon Valley. They are beautiful too but I just don't use them as much and I should. Also I love what one player said about Appaloosa Plains being the definition of Fall. I can really see that! Should play there more too.