Even though I enjoy the Dawn War Pantheon, either Temple of the Silver Flame or the Blood of Vol are probably my favorite official D&D religions.I have an appreciation of the Temple of the Silver Flame and the Blood of Vol as religions / faiths. Having undead being not necessarily inimical to the average peasant, even gilded with a type of nobility, is an interesting take. It blends well with the city-state of Hollowfaust from the Scarred Lands. (Although that's a culture and not a religion.) The Silver Flame I enjoy as it has a complexity that I could see existing in the real world, as mentioned above. I've read some short stories that may have inspired it, so that's also a cool little novelty there. Now that I think about it, the Mithril city-state from the Scarred Lands meshes well with that, too.
Wee Jas is maybe my favorite deity in Greyhawk. You may already know, but there is an answer. Wee Jas was originally just the goddess of magic (and law) for the magocratic Suel Imperium; however, after the Rain of Colorless Fire destroyed their empire, the distraught Suloise people began desperately looking to Wee Jas in concern about their dead. So Wee Jas took on that mantle, becoming a caretaker of the dead for the Suloise people.I have a strong appreciation for Wee Jas from Greyhawk as she strikes me as a very syncretic deity. Death, Beauty, Magic I think are her spheres of influence? What happened in history for that to come about?
This is his personal intepretation, but I personally like the idea that the Raven Queen is actually Wee Jas. Since the history of the Nentir Vale is not the same as Greyhawk, Wee Jas could have very well once been the haughty sorcerer-queen that Nerull renamed to "Nera" who then became the Raven Queen or maybe the whole ascended mortal story is a facade to conceal her true origins."Most of the Raven Queen-related lore is my baby [...] I interpret the Raven Queen as basically being Wee Jas, albeit having absorbed Nerull's power and become a blacker deity".

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.