I usually homebrew, as do many of the DMs around me. However, on occasion we've delved into non-homebrewed worlds, and here is my faves.
To Play In: Planescape. Man, it was so cool and I loved the slightly off-kilter feel to the planes, esp. Sigil.
To Read: Ravenloft. I didn't mind small stories set there, but I'm not sure I could make it a campaign. However, I could read the novels and old game-books for hours...
To Run: Eberron. Its 20th century feel was a great relief from sword-n-sorcery. Its also one of the most logical settings I've seen. I so wish there had been a few more supplements before the 4e switchover (and the subsequent 3-books-done method of handling settings).
Honorable Mentions: Golarion is a setting I keep meaning to learn more about; gonna need to get the Inner Sea guide sometime. Likewise, I REALLY wish I'd have picked up the Gazetteer of the Known Realms (Aerth) before it went OOP. Both of those worlds do a good job of presenting classic D&D worlds that are perfect for adventuring in.
I'd be remiss to not mention Mystara, for it was my first setting and my Homebrew is built on its skeleton.
LEAST FAVORITE: I never got into Dark Sun, since I don't really like the survival/grim situation. Likewise, Midnight's doom-and-gloom never was my cup of tea. However, the honor for least goes to Kalamar; ye gods, the Kalamar books were some of the most BORING book's I'd ever read. It was like reading the Encyclopedia! Top that with unpronounceable names and the flattest pantheon I'd ever seen, and I couldn't get 5 seconds into any book produced. I seriously couldn't believe it was done by the same people who do KotDT and Hackmaster...