Quake 1
I received a new PC, all chuffed and protective over it and along comes my brother, wants to install something on my. The audacity of it!
But he persists, talking about true 3D, being able to look up and kill zombies....and I eventually give in...he installs Quake.
Needless to say, the following week I go to work all red eyed and tired, game gripped me like nothing before or after. I spent hours on ftp.cdrom.com downloading mods, maps, models, etc.
Later on I met a friend and we played loooooooooooooooooooooooong hours of multiplayer.
To this day I still have Quake 1 installed with a ton of mods and still play it regularly.
Most epic of games!