My definite favorite is Forgotten Hollow. I simply love the aesthetic and while I would have loved it to be bigger, it works for me as my dark corner of the woods. (In comparison to it, Glimmerbrook really disappointed me. Even more so after I saw all the nice BB assets. I'll admit I upgraded the existing homes in Forgotten Hollow, but the houses in Glimmerbrook feel so bare and incomplete it is kind of ridiculous. The world does not really stand out either. I know it is supposed to be the "normal looking neighborhood with a secret" thing, but the normal neighborhood has to stand on its own and it doesn't. Compare it to smaller Forgotten Hollow that does pull of an interesting vibe.)
My runner up is Sulani. Absolutely gorgeous and really fun for off the grid and beach play. I often times find myself in a household there.
It is difficult to pick a third. For play time, it would have to come between San Myshuno and Brindleton Bay. Both solid, fun worlds with independent appeals (pets and lighthouse v. festivals and city life).
The rest usually have something appealing about them, but nothing major to me. Willow Creek and Oasis Springs feel really basic; I often overlook them even if they are solid neighborhoods with secret areas. Widenburg feels pretty basic, too, honestly. I also never seem to have a house ready for a larger family there. I do really appreciate the size of these three, though. Same with Newcrest, which is all about endless possibilities (which as much as I adore, takes a fair bit of commitment and the aesthetic is on the plain, boring side).
I never use Magnolia whatever from Get To Work as anything other than a public lot world anymore, though I used the one free lot for a while. I also do not really count the vacation worlds myself, if only because I cannot grade them like the rest, so Granite Falls and Selvadora are out.
Del Sol Valley is kind of interesting for fsme play, but you have to build a house for a larger family or move into the celebrity neighborhood. I actually like the Strangerville world fairly well, especially the wealthy neighborhood on the hill. I usually put the fancy restaurant from Dine Out and a Spa up there. Britechester feels more like temporary world rather than a home (but my college town felt like that, too, so no hard feelings). Evergreen Harbor is kind of cool for the new grunge and the development N.A.P.s, and I have been playing there, but beyond the ready placed community lots and the newness nothing is keeping me there.