Just been thinking about the games I've played through my lifetime which have had the greatest impact and lasting memories on me;
Rebelstar (played this game when I was very young, the first game I really loved)
Half Life (goes without saying)
System Shock 2 (fond memories of this game)
GTA Vice City
EVE Online (would'nt like to calculate the amount of time I put into this game)
Elder Scrolls Oblivion
Those are my standout games spanning around 26 years of gaming.