I'm a little biased as I'm OLD... but I'd say that on my list of top 5 would be:

1. World of Greyhawk- Hard to argue with the original "fantasy RPG" setting. I've played and written there for almost 30 years and I'm still in love with it.
2. The Land (from the Thomas Covenant series) amazing history and richness. I realize it's very "middle earth" but it does have it's own soul and character.
3. Middle Earth- Of course the classic and father of them all. I read the Rings trilogy when I was 10 and they are still some of the best books I have ever read.
4. The Kingdom of Shendenflar- Never heard of it? Well that's because it's my own campaign setting. I've been working on it for nearly 25 years and it's hard not to love something you've been with for that long. Sure it doesn't have the sweeping "epicness" of Middle Earth or any of the others but there is a lot of history and adventure there.
5. Faerun- The middle child of fantasy RPG settings. Epic in scope and detailed unlike any other.