One of the most used adventures from my AD&D 1e days is Under Illefarn. Although set in the Forgotten Realms, I've placed it in Greyhawk and beyond. Micro setting, dungeon, and adventure seeds, there is little more that you could ask from it.
In truth, N5 - Under Illefarn is probably my favorite AD&D 1e adventure.
In PDF, Under Illefarn is on sale for 3.50 (normally 5.00), and is now in Print for 11.99 (print plus PDf for 13.99)
Written by Steve Perrin, it is truly a treasure.
Daggerford is fairly quiet for a frontier town. Sure, lizard men raid the local baron's holding now and then, orcs sneak out of the Misty Forest to raid caravans on the Trade Way, and Lady Bronwyn has a few suitors who are a touch too passionate, but there's nothing going on that a member of the renowned Daggerford militia can't handle.
Except that the Shining River has turned green, and cattle are dying. Except that the Elf King of Laughing Hollow, a place where no human dares go without fearing for his life, is asking the militia for help. Except that strange substances are oozing from the fissures caused by a recent earthquake. Except that the earthquake also has opened an entrance in the cliffs around the Laughing Hollow that might lead to the fabled dwarf mines of Illefarn.
Suddenly, being a militiaman isn't quite as easy as it used to be.
"Under Illefarn" is the first module produced for use with the Forgotten Realms Campaign Set.