My favourite published campaign setting would be...

[Darrin Drader]

I used to really enjoy the Forgotten Realms, but not so much anymore. I ran a couple games set in the realms during 3rd edition, but I began to feel like it was too hard to create an original concept in that setting without stepping on somebody's toes. Of course this could be overcome by simply agreeing to not worry about continuity, but that carries with it certain feelings of not being true to the setting, which is stupid, but it's always there and I know I'm not the only one. That said, I'm happy I was able to add some little bits to it here and there.

The setting I'm currently running is Golarion. What I like about it specifically is that there are a lot of regions within the setting, so it's not hard to find a spot where you can do something new without becoming entangled in previous lore. Of course as Paizo works to fill in the gaps, this might start to become a consideration, but it's a long time before it will be at the same point FR has been at.

I like Goodman Games' DCC #35 simply because of the fact that they aren't in any hurry at all to fill it in. While the one box set presents a world that is fairly generic in flavor, it has endless possibilities for customization.

Finally, I like historical Earth. I started a thread earlier on using Crete circa 1,500 BCE as the backrop of a campaign I've tentatively called "The Minoans."

 

[Crothian]

Thieves World is my favorite published setting. I don't get to use it that often, but that doesn't make it any worse.

 

[WayneLigon]

Dark Sun - That is, there are many ideas and concepts I love in Dark Sun, but the execution was almost comically clumsy. I would very much love a 'sword and planet' setting done right.
Eberron
Freedom City
Blue Rose
New Pavis or the Orlanthi areas of the Runequest world
The Pathfinder world is interesting but I've not read the gazeteer yet; it might turn out to be a favorite.

 

[HelloChristian]

Gygax's Greyhawk. Everything I needed in two little books and two poster maps. It's amazing how much gaming can be created from a relatively small (by today's standards) supplement.

 

[Remathilis]

My personal favorites are Eberron, Ravenloft, and Planescape. They are the only three "settings" I've run that I didn't design. Notice, they are all quite different than Greyhawk/Realms style play; that's intentional. If I want that, I run my own world. If I want something unique, I run something else.

 

[El Mahdi]

Fantasy: Forgotten Realms

Sci-Fi: Star*Drive

 

[cougent]

Finally, an easy question for me to answer!

The only "published" setting I have used in D&D (AD&D actually) was Greyhawk. I ran 3E entirely in my own HB setting.

During my hiatus from 2E, I did love the concept and world of EarthDawn also.

 

[SpiderMonkey]

I went on a homebrew bender for awhile, but now I find myself drawn back to published settings.

Favorite:

Known World/Mystara

Runners up:

Pre-2e Forgotten Realms
4e Forgotten Realms

 

[DEP]

My favourite campaign settings would have to be:
Gygaxian Greyhawk
Golarion
Eberron
followed my Mystara and Dark Sun

 

[teach]

1. Planescape
2. Dark sun
3. Al-qadim
4. Ravenloft
5. Etherscope

I think I like settings that take a pre-existing ruleset and setting, and try their hardest to take pre-conseived notions and flip them on their heads.