My favourite published campaign setting would be...

[Aus_Snow]

While I'm going about adding to lists, I might as well to this one too. There's an important one I missed:

Westeros (as detailed in the 'A Game of Thrones' RPG from Guardians of Order)


I'm waiting and hoping, however, for even this superb treatment to be eclipsed by that given in the 'A Song of Ice And Fire' RPG, which is soon to be released by Green Ronin, last I heard.

 

[avin]

1 - Planescape
2 - Mage: the Ascension
3 - Deadlands
4 - GURPS Voodoo
5 - GURPS Supers

 

[Betote]

For D&D Fantasy, I used to run only Greyhawk, but now Golarion is my setting of choice. What I like the most about these settings is their openness and that each region is unique in some way or another, so I have a place for every story I happen to come upon without feeling restrained by an overwhelmingly detailed background as in Forgotten Realms. I'm quite fond of Scarred Lands too, but I just don't have enough time and I need to focus on just one fantasy setting.

For non-strictly-D&D Fantasy, I like Eberron and Ravenloft a lot. If I'm not running these more is because they might be too focused and quirky for the amount of variety I want in my campaigns.

And for not-D&D, not-fantasy, The 40K universe is very attractive to me, as are Deadlands, Freedom City and HP Lovecraft's Providence.

 

[Betote]

There're, also, settings I love reading about but which I find unplayable. The two main offenders here would be Westeros and the Middle Earth. These settings have such wonderful stories carved in that I feel that the PCs don't have much to do without shattering what I like about them.

It's the same feeling I get from Forgotten Realms, although I don't care for the Realms at all

 

[DaveMage]

Faves (Alpha Order):

Forgotten Realms (TSR/WotC)
Oathbound (Bastion Press)
Planescape (TSR)
Wilderlands of High Fantasy (Judges Guild/Necromancer Games)

I haven't had time to dive into Golarion yet, but I've heard good things.

Others I like a lot:
Aereth (Goodman Games)
Greyhawk

 

[lmpjr007]

1) Ravenloft
2) Dark Sun

 

[CleverNickName]

My favorite has always been the default game setting introduced in the Expert Rules Set. I think it was eventually called Mystara, but back when I was collecting all of the gazetteers for it, we just called it The Known World. I even liked the Savage Coast and Hollow World expansions, but they never really caught on.

Anyway. Mystara for the win.

 

[Invisible Stalker]

The 13,000 comic books suggest that DC is my favorite published setting.

For fantasty role playing, old school Gygaxian Greyhawk is my setting of choice.

I also really enjoy Freedom City for M&M.

 

[Khairn]

hmmm .... this is a tough one. So Ill start with the easy one's first.

Top 3
Iron Kingdoms
Harn
FR - Pre 4E

Then come the honorable mentions

Shadow World (ICE / RM)
Midnight
Birthright
Warhammer
Planescape
Golorian
Scarred Lands
Spelljammer

 

[jdrakeh]

Harnworld (Lythia) without a doubt. I, too, like Medieval-ish fantasy settings, but I prefer them to be internally consistent and logical. No D&D setting meets that criteria for me. Harnworld, OTOH, nails it every time.