Best D&D Settings: Planescape and Dark Sun. Plenty complex, rich, and detailed. It could support low-level adventurers as well as high-level power players. Both settings had plenty of room for introducing new elements, and Planescape was practically Mulligan Stew, if you wanted it to be.
My first Planescape game had a Mythic Era Klingon in it, scale mail and bat'telh included. He did not in the least seem out of place.
Worst D&D Settings: Birthright and Dragonlance. Birthright seemed to focus too much on bloodlines and weird powers from your ancestors, and power-politics should be a game for high-level D&D characters, not your fresh-out-of-the-box first level characters.
Dragonlance just doesn't allow you to do anything. For plot there is the War of the Lance or... nothing. And if you choose the War of the Lance, you can either replace the Heroes of the Lance or you can... nothing. The books were great fantasy fiction, but there is absolutely no reason to play this game.
Best non-D&D settings: Gamma World and Shadowrun. Oh, and Deadlands.
Worst non-D&D settings: Luckily, I've never played a non-D&D game that sucked.