Best Settings:
the world of shadowrun -- low fantasy meets high tech, with some amazing metaplots that drive the entire game forward. Pick a year between 2050 and 2063 to play in, and there's so much going on all over the place, it's just amazing.
Dark*Matter: I didn't like Alternity's space opera trappings in StarDrive, but when I cracked the cover of D*M, I was hooked. I'm working to bring this into a d20 modern game.
The Weird West: I really liked the world of Deadlands. Didn't like the system too much, nor its d20 incarnation.
Worst Settings:
Greyhawk: Boring. It's peak was before my time as a gamer. I can't conceive of it as anything but a 1st Edition world.
Forgotten Realms, 2E: I got really sick of all the real-world parallels brought to the world: The Horde, Maztica, Marco Volo. A little more orignality would have been nice. I haven't read the 3E stuff yet, and can't stand the novels, but I hope it's taken a turn for the better. I really loved the 1E boxed set, and the "core" FR products from 2E. I just hope they don't announce a gold rush and turn Icewind dale into Jack London's Alaska or drop a wild west setting onto the Maztican continent somewhere.
Ravenloft: Blah. When I want gothic horror, D&D is not what comes to mind. This product line really overextended itself.
No Opinions:
dark sun: I've got a few of the products, never read them. Kinda liked the concept.
Planescape: loved the computer game, never read the game product. Don't own any of it.
Kalamar: waiting to receive the campaign book I ordered to see if this is going to be the home of my next campaign, or if I'll end up using...
Mystara: Very interested in the feel of this world after I cracked open the boxed set I got for $5 at Kay-Bee toys about 5 years ago. Was looking through my stuff to figure out where to set my next D&D campaign (homebrew bored me).