The best game for a long campaign is the one you (and your players) can keep generating content and interest for. If that's D&D, Mutants and Masterminds, Traveller, or anything else, I think you're good.
We've had our longest campaigns with D&D, various editions, because that's what we keep coming back to and keep generating new interest for. I think the fact that it's SO VARIED in monsters and settings (dungeons, towns, wilderness, planar escapades, etc), as well as readily plunderable published material, that it lends itself well to keeping things fresh.