I remember coming back from school and playing call of duty(first one), I was even leader of a clan. I have also good memories with Guild Wars when it comes to multiplayer games.
But most nostalgic games are for me commodore c64 ones and PC games from mid to late 90's. I almost cried when last year I stumbled on youtube on soundtrack from Last Ninja from C64. Last time I heard that music over 20 years ago, and yet I recognized it immedietaly. I remember playing Last Ninja 1,2 but it was way too hard for me back then.
I remember mid 90's, summer... I was playing first X-COM game soo long, whole night. When it was morning I thought it was still the day before. All that time till ~2002(middle school) was magical for me, so many other great games like Fallout 1 & 2, Planescape Torment, Jagged Alliance 2, Baldurs Gate 1 &2.
2002 was maybe the best year in terms of video games: Mafia, Gothic 2, GTA Vice City, Morrowind, Jedi Knight II, Hitman 2, Icewind Dale II, Medieval Total War, Battlefield 1942, Neverwinter Nights, Age of Mythology, Tony Hawk Pro Skater 4 and more... all those games I spent so many hours in, and all that was so long ago...