Wolf3D
Descent
Quake 1
Super Mario Kart
Gran Turismo
reasoning
Wolf3D - was the first FPS i ever played..
Descent - first proper 3d movement game i ever played, being able to fly through tunnels that go up, down, left right, it blew my mind away
Quake - first 3d Accelerated game i played and opened up the 3D market
Super Mario Kart - it made racing fun, its still damn fun to play when im drunk with me mates :P
Gran Turismo - it brought me into the world of proper racing and what cars can and cant do.. for its time, the physics where amazing..