I don't mind playing RTS and FPS against friends (either local or friends I know overseas). That's always a lot more fun than just playing the A.I. all the damn time.

MMO's are pretty cool, well I love the group feeling of FFXI, most people I come across seem to be rather mature (maybe that's because I'm in Japan's time zone, so I miss a lot of the Americans). There's also other MMO's that have a more mature player base, and gameplay.

One that comes to mind, and that I played in beta for awhile, was Uru Live: Myst Online. Absolutely beautiful worlds and grouping together with mature people to solve puzzles and explore, is a pretty good feeling. Sure the games offline are incredible too, but you can't get the same feeling as Uru Live.