As for the best campaign setting, I don't have a personal favorite and won't because each setting will get modified in some ways by most DM's anyway, and each of us like different things for different reasons.
Me, I like Scarred Lands over Kalamar simply because its darker and grittier as it is written. There are some things I do like about Kalamar more, like the written alphabets and languages (that is really cool) which is stuff I think greatly missing in many sourcebooks.
I put Forgotten Realms in the middle because I am so used to it and I like the setting because sometimes its just fun to play in a high magic game that is more lighthearted (but it can have its gritty side to it, read Lords of Darkness for some of the darker aspects of the world).
that said, I have seen more anti-FR sentiments on these boards than any other settings, and I don't have a clue why, but I guess that is because its been done amost to death and many of you are probably tired of it and want something new, which I totaly agree with.
I put Ravenloft down further on the scale because it just doesn't appeal to me at all, period. Al'Qdim (I know i misspelled it) isn't one I like a lot either, but that could have been because of the DM that was running it and my character died in the second session because he set me up.
However, Gamma World, now that is just plain fun and equal in fun for me as FR is. Sometimes there is nothing cooler than a mutant bunny with a flamethrower riding a old jeep across a vast wilderness running away from some stupid pure humans that think they are superior just because the bunny has long ears and a bushy tail driving a jeep.
Then you got Dark Matter, probably one of the best non-d20 settings ever, and for me personally the coolest non-d20 setting that needs to be converted over is Torg, which is a blend of everything in one very messed up and chaotic world. Okay, so I am pushing and hoping it gets done, nothing wrong with that, is there???
That said, I am getting off the soap-box and the rant and am going to sleep cuz its late and I'm getting tired. Just remember that each has his or her own personal likes and dislikes, and that is what makes gaming so great.