Team Fortress 2, with over 2100 hours logged. More back on the Xbox 360 copy and when it was free during the Mac Update.

Team fortress 2 for me

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.
• Mindustry is a blast

There is a soft spot in my heart for SuperTuxKart too, despite the current project leadership.