I thought I'd have to think quite hard about this because I tend to play long campaigns, but it turns out Pathfinder wins fairly easily - even if you take the Interlude characters we're currently playing as seperate from the main Pathfinder game, I think it's still just in front.
The Awesome Uni World of Darkness campaign I played definitely needs a mention. I was still in the first year of my three year degree when I joined - I'm not sure how long it had run before that, a year or more, maybe? The game wrapped up before I finished uni, though I forget exactly at what point. I certainly played for at least 2 years, probably nearer 3, and if we were taking game world time this would be the one that won hands down because I joined Vampire in the Dark Ages - Sabbat styley. The others had played generations in both Vampire and Werewolf from Roman times and had recently played through Dark Ages as Masquerading vampires and reached Modern Day in Werewolf (we all much preferred Werewolf - something about the intimacy of the pack versus the mistrust of the cotierie, I suspect). Once Vampire reached Modern Day, we added Hunters to the mix, including some players who weren't involved in the other games and that was a lot of fun.
After uni, we had a regular roleplay group that fell to pieces and reformed in various forms on various occasions. We'd settled into a fairly steady pattern of alternate Sundays and running short campaigns in different systems when Rich took the reigns with Pathfinder. He wanted to run the Kingmaker campaign, so we created Brevic characters and went with it. I'd recovered from the aversion to D&D-type games my very first roleplay group had inflcited on me (it was something they all agreed on: D&D was too clunky and too overpowered and I should never bother trying it because it was just rubbish); their influence still colours my opinion (it is clunky), but I've found the right group and/or the right storyline means this is not insurmountable (maybe this is why I'm more forgiving of 4th Ed than D&D fans seem to be?)
And the Pathfinder game turned out to have a very powerful storyline, even if 3 or 4 years on (or longer...) we still haven't technically completed the first book of the campaign we were meant to be on. We got to a suitable stopping point - defeating the Stag Lord - and Rich offered up his seat, knowing Husbit wanted to run some more Deadlands or 'Punk and there being a few other games being mooted - but we were enjoying this game and the other games weren't quite ready so we carried on and sort of forgot about switching...
In an ideal world, I'd love enough free time and disposable income for me and my gaming network to play, prepare and run all the games we want. There would be a lot more long campaigns - and I think a lot more fiction flowing therefrom, from several members of the group.