1. Team Fortress 2 - So many hours of fun. Been playing since release and it's still my daily game.
2. World of Warcraft -Also offers a lot of hours, which should span over 3 years. I'm not much of a player as I used to be but it's still good imo.
3. Counter Strike: Global Offensive - Really enjoy all counter strikes and I could happily play CSGO every now and again.
4. Left 4 dead 2 - Got it since release, still play it. The game always creates fun moments (only online though, single players meh)
5. Half Life 2 - Fantastic game