I've run, I consider, about six "excellent" RPG campaigns -- excellent from the point of view of both me and my players having loads of fun, telling interesting stories, seeing interesting characters develop, and just generally raising the tone to something that sometimes approaches the mythic.
I'm reminiscing, well, because someday I want to surpass these. I have many more games to run -- I'm starting one next week -- and one day I want to have replaced most of what's on this list with even better stories.
Many of the campaigns I've run don't make it to the list of excellent campaigns (the D&D one I'm running right now, for example, is perfectly adequate, but is unlikely to belong on a list like this).
The top six goes like this. (The number of players listed are the average number I had in the game at any one time; some games cycled through several more during the course of the campaign).
6. Rhodri's Folly [Ars Magica], 1998-2002. Five players. An elderly Tytalus brought young magi into a covenant that he'd arranged to be in the most pressure-laden and inconvenient place possible, then promptly vanished into Twilight, leaving the youngsters to fight off political opponents, make peace between the fey and mundanes, and incidentally realize it was up to them to save the world. An experiment in making a totally absent villain, and it worked very well at that level. A live-action Tribunal that I wrote on my own featuring 15 guest stars was both a high and low point; the Tribunal on its own was wonderful, but it left detritus in the game we had to agree to disregard. Petered out early, but had some very good times first.
5. River of Cradles [Runequest], 1995-2000. Eight players. I ran nearly every published adventure on the River of Cradles, only missing the Cradle scenario itself (we moved on into D&D 3E and didn't reach this natural climactic scenario). While everyone had reached Rune level and exposed the limits of the RQ system, it was still my longest-running and favorite Gloranthan campaign to date.
3. [tie] Caer Glamorghan [Ars Magica], 1992-1996. Six players. My first and by far best Ars Magica campaign. Also Welsh, quieter and more contemplative than Rhodri's Folly, with several long-running story arcs. Walked through many published adventures; I did extensive amounts of adaptation to make the stories flow into one another and involve the characters personally. Very family-oriented, probably partly because Mike and Susan, and Sherilyn and I, were newlyweds at the time. Some of the most memorable characters I've ever had the pleasure to GM -- Sionna the witch with eight-and-a-half children; Meredydd the faerie and her straight-laced German Criamon magus husband; Tristan the shuffling, nearly diabolic Criamon... I was very proud of some closed time loops and regio work throughout this campaign.
3. [tie] Donner Prep [Champions], six players, 1996-2001: Easily the most pure-fun game I've ever run -- I eclectically and shamelessly stole from everywhere. Consciously intended to copy early New Mutants, set at a superhero-training school a few miles from Lake Tahoe, the teen heroes fought a forest fire, traveled dimensions, took a four-stage time travel journey that saved the world (and their school) twice over; visited a Dyson sphere... all while debating philosophy with their Heinlein-inspired professors and dating members of the Sacramento-based paranormal-mercenary group (inspired by Before Breakfast).
2. Toiyabe Caern [Werewolf: the Apocalypse], four players, 1993-1997: An intensely melodramatic game. The best single moment I've ever GMed for came in the White Wolf Vancouver setting... where the pack of werewolves, for the sake of a friend they had no reason to believe could ever be saved, stayed and fought instead of walking away, knowing it would destroy the truce between Kindred and Garou that made Vancouver unique (and peaceful); also a star-crossed Romeo-and-Juliet doomed romance, and an Umbra-spanning search for Native American spirits and elder Garou allies to establish a new caern.
But it's obligatory -- the best RPG I've ever run has to be:
1. Nora's Home For Wayward Youth [Mage: the Ascension], eight players (1992-2001): Jeremy was the one who made me realize how special this was, when he described another game he was in at the same time: "Well, it's fun, and I'll keep showing up, but it's not like it changes my view of what roleplaying is." I am privileged to have guided this story -- I often didn't have to do much but sit back and interject comments into the cacophany of the players debating, in character, the nature of the reality I'd helped them develop. Earl was a tremendous help throughout, since he played a character who seldom left their chantry home; as a story would weave from scene to scene, half the players would be talking with me developing the scene, and the other half -- their scene having ended -- would be gathered around Earl, trying to figure out what had just happened. Some episodes were exquisitely roleplayed: the incomplete rewrite of history that erased Martin; Slade's coat-quest; Scooby Doo In Tyrolia (the discovery of the tomb of Ares); the Love Boat In the Bermuda Triangle (a farcical boat ride with every sort of paranormal imaginable); the introductions of Chryse (walking through the lands of the dead) and Christy (the best job I've ever done at keeping a horror-movie atmosphere); and several more great stories. Not every episode was so magical, but many were, and the way it clicked was at times beyond what I thought roleplaying could be.