For me, its probably Command & Conquer Red Alert 2 - Yuri's Revenge (2001).
I still occasionally play GTA San Andreas. That game was my childhood.
If we include series then total war since I'm only playing recent games. I recently tried Eden eternal again as it's my most played game but I couldn't get back into it.
The oldest game I play is TF2. It's not even an old game in the grand scheme of things (Oct 2007)!
I'm replaying Metal Gear Solid (1998) atm.
Age of Empires 2 and Phantasy Star Online (The Dreamcast version)
I play PSO 3-4 hours a week there is a big meetup every week. Age of Empires I have played a lot recently with some friends because its hilarious sending you're troops to slaughter you're friends workers. xD
- Pokemon Red and Blue (1998)
- Worms World Party (2001)
- Jak and Daxter trilogy (2001-2004)
- Ratchet and Clank original trilogy (2002-2004)
- Persona 3 (2006)
- Spore (2008)