Red Hand of Doom - 3.5 Edition D&D
Red Hand of Doom showed just how far D&D writing had come. Instead of a linear adventure through yet another dungeon, it tasks the players with defeating a horde of monsters coming to wipe out the villages and towns in a generic Vale (it's left ambiguous so the adventure can be used in any setting or campaign). The party can use guerilla tactics, spy on the advancing horde, venture into a lich's lair to deprive the horde of its undead minions, and finally take on the horde itself, with their preparatory actions coming back to aid or haunt them. It's a superb adventure, and one that any D&D fan should play.