Greyhawk is my favorite setting, but it's important to note that there's several Greyhawks.
Gyagxian Greyhawk is pretty much the 1983 Boxed Set, plus the 1st Edition Adventures written by Gygax and a small handful of others. This is popular mostly with grognards who liked Gygax's work, and felt that the changes made after his departure were awful (and often spiteful). This notion is a static setting, with CY 576 as a starting point for every campaign, but the work of the DM and Players would quickly make it their own. There is no real notion of "canon," because only the information is that which is given in the Folio (and even that can be considered inaccurate in game due to the way it was written).
The Greyhawk Wars began advancing the story of the world, which gave the notion of canon for the setting. It advanced just like the Realms did, and new products were made to show the changes to the setting over time. This was mostly the 2E era, but the 3E Gazetteer did a good job of continuing this.
The Default Greyhawk of 3E was an abomination. It was the death of the setting, IMO, since the setting didn't mean anything anymore (or more accurately, it meant EVERYTHING). This should generally be disregarded as a real setting, just as the Known World was in BECMI (before it got turned into Mystara).
Living Greyhawk is technically non-canon, as it's the RPGA setting that allowed for non-employees to create content for the setting. The 3E Living Greyhawk Gazetteer was a pretty good product, however, and is much more detailed than the 3E Gazetteer. Additionally, there are hundreds of adventures made for it, but they can be tricky to get now that the organization is defunct.