I'm partial to, uh, Zagyg.
Other than the obvious, these are some of my faves (no particular order):
Fharlanghn
Heironeous
Hextor
Incabulos
Istus
Nerull
Pholtus (as a jerk)
Trithereon
Wee Jas
Xan Yae
Zuoken
Celestian and Trithereon are my favorites, but also a fan of Wee Jas, Ehlonna, Heironeous, Istus, Mayaheine, Phaulkon, Rao and Xan Yae. Plus a lot of Roger Moore's 'demihuman dieties,' like Aerdrie Faenya, Arvoreen the Defender and almost all of the Gnomish pantheon! (Loved how the Greyhawk book for 3rd edition had Istus grant both the Chaos and Law domain options. That's just fun. She's the goddess of pre-ordained fate and random happenstance! Suck it up!)
And have a fiery loathing for Sehanine Moonbow, for no rational reason whatsoever. It's not that she's 'new,' I like some 'new' gods like Mayaheine or the halfling death-god Urogalen.
While, in other settings, I'm a fan of some of their evil gods, like Auril in the Realms, or Belsameth in the Scarred Lands, I do not care for any of the evil deities of Greyhawk. Hextor is about as tolerable as it gets. The rest are either utterly mad like Tharizdun, or utterly silly, like Wastri, and I can't find a use for them, or their followers, narratively.