Ghouls N' Ghosts
preferably the arcade version but the Genesis and Supergrafx ports work too. I don't play it everyday or anything, but when I do fire it up, I never regret it. I also never feel bored while playing it.
Runners up would run into the dozens if not hundreds. For all the bitching I read (and create) on the internet, there absolutely boat loads of awesome games in the world. Bioshock 2 is the awesome game I'm currently spending much of my time with.