Honestly, there are far too many...
My first introduction to games was the NES, which I really enjoyed. At age 6, the games were fun but they never really blew me away. 2 years later, my family got a PC and I never touched the console again. I have to give the honor of most memorable game to Descent though, simply because it was my first PC game. I played it to death at the time.
I already loved the game, and then something magical happened - my school got a computer center, and the only multiplayer game there was a demo of descent 2. In that microcosm, I was a champion - not one of the best - THE best, and by a long shot too - those other kids were just learning the ropes, but I had completed both descent 1 and 2 multiple times. It was the only thing I really good at at that age, besides drawing demons.
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There were so many other games though - initially these were Doom, Heretic, Duke Nukem 3D... but the list just goes on and on..