Forgotten Realms, especially since 3E came along, as the villains aren't treated like jokes anymore. The Meeting of the Archmages features in Dragon had a lot to answer for imho. Having Elminster and Mordenkainen sit around and joke about the Zhentarim, cos yeah, it's really funny to laugh about people who enslave, murder, blackmail, extort and steal as a daily routine. Love the world Ed Greenwood created, but he is far from the best writer who has developed it.
Ravenloft, was very impressive before 3E came along, and more so now with the much needed Gazeteers fleshing out the domains (though apparently theres been some kind of argument between the developers and the writers of that series and annoying we're not likely to see more of them).
Al-Qadim, had the most consistent level of development on it, even though it was a limited product line. Not a dud book that I can point too, and the Ruined Kingdoms box just rocks, especially the uber-bitch Tisan Balshareska!