Day 1 Can be found here.
Day 2 Favorite Playable Race
Day 3 Favorite Playable Class
Assuming we are discounting home-brewed settings then I will rely on the limited experience I have with published campaign settings. Dark Sun has always held a special appeal to me, very Science Fiction with my favorite dose of combat: gladiatorial. My experienc is extremely limited so I am hesitant to call it a favorite. Likewise with Greyhawk, while I adore the modules I've never felt an interconnecting atmosphere within it and a gameworld is more merely the content of its dungeons, as magnificent as they may be. Dragonlance is a great setting for reading but I've never felt the need to play in it.
I've played a lot in the Forgotten Realms of Faerun and I've read quite a
bit of pulp on it. That said while the Baldur's Gate series does a
fantastic job at making a memorable setting I've had nothing but trouble
running it and playing in it. I'm afraid it is a setting that is
simply too fleshed out, there is little room to go without tripping over
something already existent.