Greyhawk
Old-ass setting made with help from the G-man himself, lots of the core fluff is from here.
This setting is by far the best one, Toril can go fuck itself because Oerth is THE SHIT! None of that cheery idealistic bullshit from the forgotten realms flies in greyhawk which is cynical and gritty. A peasant's life is full of shit, just like it should be. Magic items do not grow on trees in Oerth. The Gods are not going to help with every little problem you have here. The Blood War's origin is actually clear and logical. And the Circle of Eight are total badasses.
Up until 4th edition, every edition change's fluff explanation was the result of some great event in the greyhawk setting. For 1st to 2nd the explanation was a great plague with mysterious origins that was altering the fabric of reality and magic itself. For 2nd to 3rd, the explanation was really, really awesome; Vecna proved once and for all that he was the end-all, fuck-all of fantasy villains (fuck you Sauron) he was trapped in Ravenloft as one of the Dark Lords, but he did what no dark lord could ever do, he told the Dark Powers to go fuck themselves, sliced off their balls with his magic dagger, then escaped Ravenloft to his native universe and became a demi-god. Then he managed to kick Iuz's ass and take his power to become the mightiest Greater Deity to have ever existed and tore reality a new asshole to enter the planescape setting, kick the lady of pain's ass and storm the sigil; then break reality really really hard. Even though he was beaten by a plucky band of adventurers, the damage was already done and reality was broken so bad that all the mechanics changed and fullcasters became totally overpowered, Just as planned. In addition, he managed to come back once more as a lesser deity (which means that he still got a net gain in power level.) Double Just as planned.
