As with other questions we’ve had this year, my response is “All of them?” A more precise answer would be, whatever one you have on-going material for.

I’ve played a lot of long games, but they usually last about two to three years before they run out of steam. The longest campaigns were ones where the world and the PCs changed from time to time. The Star Trek campaign I ran from 2000- 2005/6 was actually three tightly connected campaigns, one taking place coterminously with another, or a follow on “series.” I had a Babylon 5 game that lasted about three years, and a Stargate one that had a similar run. The first ended when the story did, the second faded away. The longest continual campaign was probably the recently ended Battlestar Galactica game, which eventually knocked all other games out of rotation as it powered along for five years despite players coming and going, births and other life intrusions.

I think the question is “What can you keep fresh?” The BSG game was fantastic, fun, and came to a very satisfactory end. But I miss it sometimes and keep thinking there were things I could have done with it.