For me it is a toss-up between Ravenloft and Dragonlance. I'm not just going to leave it as that, because your outraged howls of protest smote my ears like thunderclaps before I even clicked 'submit reply'.
Ravenloft is a roach motel, PCs go in, nothing comes out. Any setting that is a one-off PC setting is by default a loser and waste of time. If you can't play your main PC there, why bother?
Dragonlance is a marvellous series of books, and I've loved every one I've read. But as a setting for D&D it is an abject failure, much like Middle Earth would be. All of the important things that happen in either series of books have all been done by the protagonists of the novels. Each has plenty of heroes to go around, PCs need not apply. As a rule, I've found that epic fantasy novels make piss-poor settings for fantasy RPGs for that very reason. There's not much fun in running around slaying dragons when you know the Important Dragons are all going to be slain by someone else. The only way I could ever RP in a Middle Earth setting is if the rules assume, and state to the players, that Gandalf, Aragorn, Bilbo, Frodo, Legolas, Gimli, Faramir, Eowyn, Theodred, et al never existed, and all of the heroics they were destined to do must now fall to the PCs to do, or ME will be covered in darkness. Problem with that is, those were great stories because of the great characters. Just any thirteen dwarves and a halfling rogue setting out to slay a dragon would not have made nearly as good a tale as Bilbo, Thorin, Gandalf and Co. The same holds true of Dragonlance, what would the Inn of the Last Homely House be without Tika, who but Raistlin could challenge the Queen of Darkness, who but Tas could save them all?
Leave the books to the characters that their authors gave life.