Ah, this is a lovely question. Although I've found that a bit of wine makes most questions lovely. *ahem* On to the topic at hand:
Considering my campaign is situated in Greyhawk I use the core pantheon almost exclusively. The center of my game is Furyondy, but most of the western Flanaess is used to some degree. Here's a breakdown of the deities which have a significant part in the outcome of events:
Greater Deities:
Istus: The most subtle influence of the deities is that of Istus. A common thread throughout my campaign is the battle between Destiny and Individual Choice, with Destiny often proving the stronger of the two. Even when the PCs choose a particular path to avoid a certain prophecy, for instance, they find their very decision leading to the outcome that was offered by the oracles.
Pelor: Pelor is the patron deity of the central PC in my campaign. A sailor from a common background, but uncommon origins (Aasimar) worships the god of light and healing. While there are other gods designed for sailors, this Aasimar sails a Windtreader, one of the last flying ships left over from before the Great Cataclysm. Furthermore, Pelor is a champion of the common man whom the Aasimar seeks to defend and uphold.
Rao: Canon Hazen of Veluna plays a pivotal role in my campaign and so does the clergy of Rao. Veluna serves as the reason and calm behind Furyondy's vigor and passion. Unfortunately, Hazen recently fell ill and now ties between the two nations have grown strained thanks to the new leadership in Veluna.
Intermediate Deities:
Fharlanghn: Fharlanghn is the only deity who made a personal appearance in my campaign. In the early stages of the game travel and exploration was the theme and few seemed more suited to be watching over the heroes than Fharlanghn. The party's original cleric (who met his end in the Sea of Dust) was a priest of Fharlanghn.
Heironious: Considering the game is set in Furyondy, Heironious is a given. As the vanguard against Iuz in the north, Heironious' followers are among the bravest men in all the Flanaess. Most of the NPCs in my game are devout followers of Heironious and, as the nation of Furyondy wills it, the chaplain aboard the PC's windtreader is always a priest of Heironious.
Procan: Procan serves as more a token god in my game as my PCs hail from sailor ancestry. While none of those aboard the Windtreader actively worship Procan, all remember that their grandfathers warded themselves against the Sea God's mighty storms and fierce temper.
Tharizdun: No deity has more affect on the campaign than Tharizdun, for it is his return that serves as the meta-plot holding my four-year game together. Using his usual web of deceit and trickery Tharizdun has disguised his return in many ways so that none know that he has actually taken the place of a much feared demi-god...
Lesser Deities:
Osprem: Just as Procan is remembered with fear, Osprem is remembered with reverence. Despite the fact that the common sailors aboard the windtreader no longer travel the whale-road, all still remember Osprem with great affection and ask for her protection during their travels.
Demigods:
Iuz: Iuz is the most obvious and powerful threat to the people of Furyondy and so he is a very important deity in my campaign. The fact that he walks the same earth as the common man makes him a far more fearsome prospect in many ways. What many do not know, of course, is that Iuz is long dead. Tharizdun has assumed his mantle and seeks to re-enter the world by slowly siphoning off the remaning energies of the fallen demigod.
Vecna: Vecna plays a very low-key, but important part in my campaign. He and his followers are among the few that know of Tharizdun's subtle manipulations and have begun to quietly set others in motion to halt his actions...without letting them know their secrets, of course. Vecna is attempting to halt Tharizdun without tipping his hand and so far seems to be doing a fairly good job at it. Now, if only the mortals he's "informed" behave in the way he's anticipated...
Those are the major players, but there are, of course, many others. Bahamut, Celestian, Eadro, Ehlonna, Hextor, Olidammara, and Trithereon also have small, but important roles. Demi-human deities also figure in, but are more or less mysterious and unknown to the majority of the PCs.
