In vaguely chronological order. It's a big list as I don't really have single games which changed me in one go, more a kind of long progression
'RPG Maze game involving giant ants which I can't remember the title and isn't 3D Monster Maze' - Atari 2600
The first video game I ever played, I was about 4. My cousin who's about 7 years older than me brought it round one Christmas. We burned the scoreboard and screen border into my Granny's expensive projection TV.
Trap Door - Spectrum
Ghostbusters - Spectrum
The first games I properly played through and made progress with, and played with my friends round, aged about 8 or 9
Super Mario Bros - NES
Duck Hunt - NES
I got given a NES for Christmas, completely out of the blue and it was the most amazing thing ever. Duck Hunt had a gun you pointed at the screen!#
Super Mario Bros 3 - NES
The first game I ever completed
Lemmings - Amiga
Secret Of Monkey Island - Amiga
Turrican 2 - Amiga
Lotus Turbo Challenge 2 - Amiga
When I still had my Spectrum and NES, my friend got an Amiga. A proper computer! It was the first 16-bit system I'd seen and everything looked incredible. I spent what feels like every moment of a lifetime, but what was probably one summer school holiday round this guy's house playing these games. I still get actually teary if I watch them on Youtube, just how special that time was to me.
MS Flight Sim 3/4 - PC
I spent a year or so before I was old enough to be home alone, going round a friend's house after school before my parents got back from work. He had a 286 PC and we played MS Flight Sim lots and lots. The idea that you were able to fly around actual locations was amazing, and I still have most of my knowledge of US city skylines from this game.
Sim City - Amiga
Frontier Elite 2 - Amiga
Knights of the Sky - Amiga
F15 Strike Eagle 2 - Amiga
F117 Nighthawk - Amiga
B-17 Flying Fortress
I eventually got my own Amiga and these were the games that stood out. I've always been a plane geek and absolutely loved the varied flight sims released by Microprose.
Although I dabbled in the original Elite on my friend's PC, I never really got into it. Frontier Elite 2 however was the first game that sucked me into the universe. Before I started playing WoT it was easily the game I'd sunk more hours into than any other.
End of my lunch break at work, I'll continue when I get home