Let's see...
Some Cricket game on the Speccy with players represented by stick men (can't remember the actual name) - after having fun with that at a mate's house I kept pestering my parents for a computer. They bought me a C64 (sans tape deck or disk drive) and I ended up doing my own version of a Cricket game. Not quite commercial quality but my mates enjoyed it and it was first foray into programming, which I've been doing ever since.
Heroes of Karn on the C64 - not the greatest game but my first computer game purchase, my first adventure, and my first completed game. It also fuelled my love for text adventures which is still alive today.
Lords of Midnight on the C64, later Doomdark's Revenge too - both games captivated me with their atmosphere and scope and I've had an on/off obsession with them ever since. Many a year I planned and dreamed of working on my own variant and now, nearly a quarter of a century later, I've done just that.
Elite on the C64 - because many a session I was to be found taking on one more trip of one more mission instead of attending my lectures. That particular academic path didn't amount to much...
Championship Manager/Football Manager and Civilization series on Amiga and PC - the only series I've kept buying and playing with each new release.
Midnight/MU (an online Lords of Midnight variant) - both as a player and later a developer, the game and associated community have helped get me through some dark times in my life over the past few years by giving me something positive to focus on and contribute to. It might not have changed my life, but it sure as hell stopped it being a lot worse.
There have been lots of other excellent games along the way, but they weren't as significant, so they don't really count here.
