I'm not ordering mine, just five favorites:
Teenage Mutant Ninja Turtles & Other Strangeness - The first game I ever played. I liked the more serious Turtles of the original B&W comics and the idea of other genetic mutations living in the shadows and fighting crime.
Dragonlance/Krynn - Reading the DL novels was what got me back into RPGs after my interest started to flag. On a summer vacation, I talked my dad into buying the AD&D rulebooks and the Dragonlance campaign setting book. For a while there, I knew the geography of Krynn better than the geography of North America. Krynn was one of those worlds where as soon as I started reading, I felt like I knew the place. Sadly, I've never actually played in a DL game.
Mage: The Ascension/Changeling the Dreaming - Sort of tied. I was never into the other WoD settings as much, but Mage appealed to the part of me that likes the idea of magic lurking in the hidden corners of the world and Changeling appealed to the part of me that's a fairy.
Eberron - I like the "newness" of it. Even the Realms were well established by the time I got into RPGs and Eberron feels like somewhere that you can carve out your own legend. I like the "a place for everything" idea behind the setting and how the whole mood of the setting can change from place to place.
Star Wars - What is now basically just my online portfolio originally started out as a WEG/d6 Star Wars site. This game was sort of my "third coming" to RPGs (the first two being TMNT and Dragonlance). I didn't really get into Star Wars until high school and then got totally sucked in. I bought my first copy of the d6 rules at the Star Tours gift shop at Disneyland in a fit of Star Wars ecstasy.
Posts
Star Wars - I am running a Star Wars campaign right now based on the comic Legacy. I love it, I ported over alot of the Shadowrun 4th Ed. cyber and bioware. It has a very Fireflyesque feel to it, one PC is an Imperial Guard still in service to Roan Fel (exiled Emperor) the other a Han Solo type still quite young and very poor.
I love the feel, very cinematic, lots of comedy, and lotsof high drama as well.
Forgotten Realms- because its easy, and I know it from so many different sources throughout the years (PC games, novels, etc) that I will never run out of ideas that I can make up on the fly.
Athas: Such a compelling setting, and such a departure from everything else D&D. Greyhawk, Forgotten Realms, Ravenloft, even Krynn; they're all just variations on the same theme. Athas always felt, just different. Such a great setting.
Ravenloft: Horror is good. Horror with Magic and Vorpal Weaponry? Even better.
They tried to bury us. They didn't know that we were seeds. 2018 Midterms. Get your shit together.
Star Wars
Forgotten Realms
Mage
Vampire: the Dark Ages
Warhammer Fantasy- you can be a rat-catcher. 'nuff said.
Planescape- needs no explanation
Shadowrun. Dystopia with a certain amount of kinky fantasy.
Amber. When your setting can be pretty much anywhere and everywhere, what more could you ask for?
Paranoia. ARE YOU HAPPY, CITIZEN?
Fates Worse than Death. Dystopia x3. No hope, no future, no electricity. Makes Gamma World look like Disney World.
Tales From the Floating Vagabond. Because when you can spoof any other setting/genre/whatever, it makes it that much more fun.
I would also add 7th Sea with its swashbuckling pseudo-European setting, Iron Kingdoms' steampunk fantasy world and the Dark*Matter setting's X-Files-ish conspiracy theory filled Earth to my list. I haven't played any of them but the books are a good read.
Fallout - we played a few games in that, and the mercilessness was delightful. Usually the players are the most inhumane people.
Shadowrun - We play more punky than fantasy when I run, and I like that.
Iron Kingdoms - Definitive Steam Punk setting IMO. I just wish I knew why it never works out.
Star Wars - I try to force the punk into this, which probably makes me a bad person. The closest thing to high fantasy in space that I can get. I tolerate it because there's no good Phantasy Star esque thing, and I have friends that can't get enough of Star Wars. Still one of my favorite because of the adventures I have, and the adventures there's still room for and the details, and the character in some of the later stuff. Chiss ships!
1) Planescape
2) Mystara / Hollow World
3) Paranoia
4) Very early Greyhawk materiel. Keep on the Borderlands is the best first level D&D adventure ever written.
5) Spelljammer
6) Dark Sun
The only other setting I've played in that really seemed different was planescape, but I prefer Dark sun.
Lets see, what makes it so good. Firstly, its Fantasy Post-Apocalyptic, which makes it awesome. Then, there's unique feel of the game. Much of it vaguely seems based on human cultures, but only some of these can be identified. Then there's the stunningly awesome artwork from Brom and Thomas Baxa. I can't think of another setting that is so defined by the official art.
Its also a blast to play, last time I hosted a game, the party felt damn proud and powerful to have reached level 10!
Psionics are innate in most life, while Arcane Magic is unnatural and vampric. Gods cannot reach Athas, and all priests worship the elemental and paraelemental planes. In general the brutal, survivalistic feel of Darksun owns, there is no place for heroes.
There is some bad, namely the shittastic books from Troy Denning, in which he forces his shitty characters on the world and kills off some of the sorceror kings and the fucking dragon. I ignore that shit, for obvious reasons. He uses magical artifacts to insta-gib him of course, as it states oblviously in the "Valley of Dust and Fire", "There is no way for the PCs to kill the Dragon". One of my favourite bits of background is that the main region covered in the game is ruled by the same immortal wizards responsible for reducing the planet to its current, nearly dead state! So I ignore the novels, they are written more poorly than other AD&D books, too, which is saying something.
Well, I already play D&D like that. My favourite thing to do is steal stuff and plant it on other party members.
DnD, Birthright.
Paranoia, Alpha Complex.
Fading Suns, Err... Known universe?
Exalted, Creation.
Transhuman Space, Outer Solar System and Earth.