My favourite published campaign setting would be...

[TheSleepyKing]

If I had to pick one right now it would be Golarion, the world of Paizo's Pathfinder, which has more plot ideas per square inch than any campaign setting I've ever encountered. Every town, hamlet and privy in Golarian seems to have a unique twist and backstory and a potentially cool adventure associated with it.
(A few months ago, I might have said FR, but the boring and colorless 4e iteration has ruined it for me).

Other awesome settings:
Al Qadim/Zakhara -- Arabian nights done right.
Midnight -- ultimate struggle against the darkness.
Dark Sun -- brutal and gritty desert world.

 

[Quantarum]

If I had to pick one, it would be Glorantha (3rd edtion Runequest). There's just something intriguing to me about the world and it's cultures. The strong classical era feeling to the supplaments appeals to me.

-Q.

 

[Set]

For settings, by genre, my favorites are;

Fantasy;
Scarred Lands
Al-Qadim (particularly the 'Dark Arabia' / 'Scimitars against the Dark' variant from that Dragon article by Wolfgang Bauer)
Kara-Tur

I love much of Golarion, the (pre-Time of Troubles) Realms, (post Wars) Greyhawk and Eberron, but the ones above are my top three.

Sci-fi;
Trinity setting (you can do *anything* in that setting!)
Star Trek setting

Super-hero;
Aberrant

Pulp;
Adventure!

 

[Fallen Seraph]

• nWoD Earth (part of it is actually the mystery/what isn't known that makes it good)
• Cthulhutech (it is simply such a fun setting)
• Planescape (my favourite D&D setting)
• Calimsham (literally just that part of FR I like)
• Al-Qadim (obviously if I like Calimsham would like a whole setting that has similar aspects/more Arabian)

 

[scourger]

My favorite published setting that I have run is Tour of Darkness for Savage Worlds. The players had a lot of buy in because the genre of the Viet Nam War was already familiar from movies and family stories. It was a fun game.

A close second for me is the Alternity Gamma World setting with the Omega World rules. The AGW adventures present a great story. The OW rules are a very good, concise adaptation of Gamma World to d20. That also was a very fun little campaign.

Honorable mentions to the Greyhawk Gazzeteer for 3e and to the Judge Dredd d20 RPG.

 

[Gothmog]

Fantasy:
#1 Warhammer's Old World
#2 Midnight
#3 Arcanis

Sci-fi:
#1 WH40K universe- Dark Heresy rocks!
#2 Fading Suns

Semi-modern/Sem-Historical
#1 Deadlands
#2 Call of Cthulhu BRP
#3 nWoD Earth (especially the Mortals setting)

 

[jdsivyer]

If I had to pick one right now it would be Golarion, the world of Paizo's Pathfinder, which has more plot ideas per square inch than any campaign setting I've ever encountered. Every town, hamlet and privy in Golarian seems to have a unique twist and backstory and a potentially cool adventure associated with it.
(A few months ago, I might have said FR, but the boring and colorless 4e iteration has ruined it for me).

Yeah, I've got to admit that the more I read the Pathfinder campaign setting the more interesting it is becoming. On the surface it looked pretty standard, but when you get into it, woah, there's plenty of twists, ideas, in depth back story, etc. It's certainly gained my attention these past 2 months

 

[Wombat]

For me, there aren't a great number of settings that I really like -- I am pretty much a homebrew fan.

However, there are exceptions.

I really like the world of Harn. I have even used this setting for multiple other game systems.

I enjoy the Mythic Europe setting for Ars Magica, but that is all but cheating -- I was a medieval history major in college and heavily morph several of the visions that the game system projects on the era.

And if I am to like Mythic Europe, I suppose I like the world of Call of Cthulhu, Spirit of the Century, and other "real world with additions" rpgs. Heck, I kinda know the world

Then there are the worlds that fascinate me, even though I haven't enjoyed the game systems attached to them -- Jorune, Tekumel, Mechanical/Dream, and a few others. Intriguing potentials, but not enough to really deal with.

 

[mac1504]

Midnight would be my definite favorite. I love the entire concept, and FFG did a stupendous job with the materials (perhaps minus one book) that they put out in support of that setting.

 

[Aus_Snow]

I don't tend to use published RPG settings very often, and haven't been known to either. Even so, I've found the following ones to be thoroughly useful and enjoyable, with not an excruciating level of altering required:

Twisted Earth (Earth, post-apocalypse; the default setting for Darwin's World)
Hyboria (as found in Conan: the Roleplaying Game)
Freedom City (a feature of the main default setting for Mutants & Masterminds)

They all do what they were apparently designed to do, and really well at that. There are a few others that look promising, but as I haven't used them yet, well who knows.

Generally though, even if I like a setting, I'll tinker and hammer away at it until it's more the shape I - and hopefully my players! - like and find fitting for whatever campaign might be imminent. And/or I'll mix 'n mash. "Nothing is sacred" tends to be a guiding principle, for both mechanics and setting material, for me.