Ok, I've been a gamer since I was around 6, so here goes:
1) Super Mario Bros:
First game on the original NES which made the difference in my life. The feeling then was amazing, being to control something moving on the TV. It was..... magical.
2) Sonic the Hedgehog
Reason why I got the Sega Megadrive. Seeing this speed freak run through beautiful stages with fantastic graphics this was another memorable event in my life.
3) Street Fighter II
I was crazy over this game! I would pay and pay to play this game over and over again and would skip lunch so I can have more money to spend...... The turning point which made me a fighting game fanatic.
4) Final Fantasy VII
Great RPG game. Couldn't stop then and after that played FFVI and all the FF games there after.
5) King of Fighters 97
Only hard core SNK fans will know why I preferred the SNK games over the Capcom games as I played more of the KOF series.
Well, these are the games which really changed my life, without them I wouldn't be the gamer I am today. I'm sure everybody has a game which they remember was simply unforgettable perhaps changed the way you look at video gaming.