Best:
1) Homebrew is always best, if the DM is creative and has the time.
2) The original Greyhawk box. Just a plain cool setting. Enough of a background to give you something to do, but not so much to overload you or force you into a role. After Gygax left TSR, this world kinda went downhill, but it's still better than some.
3) Although GH is what I'd choose to run a normal D&D game in (in lieu of homebrew), Dark Sun does still call. Very intriguing and interesting setting. Well done.
Worst:
1) Spelljammer. How much crack do you have to smoke to come up with this?? The setting, even the core concept behind the setting, turns me off so much that I have nothing even passibly polite to say about it. Fantasy needs to be rooted on the ground. Tech, even magitech, doesn't fit well.
2) Planescape. Try as I might, I never did like this one. It always struck me like "Spelljammer done right". I can't really fault the setting so much. It's pretty well put together. I just prefer my fantasy to focus on the Prime, with the occasional epic jaunt to the outer planes.
3) Forgotten Realms. I bought the original gray box the week it hit stores. After I'd read through the whole thing, I said something to the effect of, "What a lame setting, it'll die a quick death." I was apparently wrong, but I still have never understood the draw. At least the 3E hardbound looks refined enough that I at least wouldn't be embarassed to play in it, but I'm still not frothing at the mouth to do so. I'll stick with Greyhawk, thankyouverymuch.
Otherwise:
1)Dragonlance. I like the setting on its own, although it isn't my favorite. The down side is that it appears it was created almost to showcase annoying races, primarily the Kender and Tinker Gnomes.
The Kender don't really bother me by themselves. It's when you throw in the gnomes that they become unbearable. That, and it seems like 75% of all players have decided that halfling = kender, whether you are in Dragonlance, Greyhawk, or FR. I've even seen someone try to play a halfling-kender in Darksun (killed posthaste).
The tinker gnomes, though, are a rant all their own. I never much cared for them in the first place. They were tolerable, but nothing beyond that. Their real crime, though, is the same as the kender. The image of a gnomish tinkerer has spread almost everywhere. To avoid a rant, I'll just say that any gnome in my game that shows any unusually strong mechanical aptitude (rogues disarming/setting traps is fine) are smote by the gods. Immediately. With much prejudice. Their very souls consumed, beyond even the reach of a wish. Gnomes are not mechanically inclined, as a race (nor are they disinclined).
