Probably Halo, but it could also be borderlands. That's a tough one...
Team fortress 2 for me
Team Fortress 2, with over 2100 hours logged. More back on the Xbox 360 copy and when it was free during the Mac Update.
If we just talk about the last few years:
0 A.D has been very fun, although I play offline because I'm casual af :) Has all the things I liked about Age of Empires and more, and I've been building from source so it gets improvements every week.
Xonotic has had an excellent community whenever I've gone online, so can recommend.
Probably Halo, but it could also be borderlands. That's a tough one...