I think you could play a new scenario of Advanced Squad Leader every other day for the rest of your life - if you could somehow get them all....
Yeah, that was an easy choice. I certainly wouldn't be able to chew through all the possibilities of my fairly limited set anytime soon.
I voted for C&C:A because it's the scenario-based game I know. But if I knew I'd always have four people, Successors is really tempting.... Also, the Ancient World series as a whole would probably win for me. At least if I get to include the upcoming titles in my playing.