As much as I hate the 2E Greyhawk, the best singular adventure that is ingrained into the setting is Vecna Lives. It starts in Greyhawk, moves west across several notable locations, and ends with a battle between Vecna and Iuz. It's a super-railroad, poorly written adventure that would have been a much better novel, but it's definitively Greyhawk.
There was another adventure in 2E Greyhawk that was a better adventure, but I've forgotten the name. The party infiltrates the Empire of Iuz to rescue a political prisoner from Furyondy. It also has a lot of Greyhawk references that makes it definitively Greyhawk.
If I had to choose a more Gygaxian adventure (other than ToEE, which is THE definitive Greyhawk adventure), I'd go with either the slavers series or the Lendor Isle series. White Plume Mountain holds the style of Gygaxian Greyhawk, but doesn't detail much of the setting.