Final Fantasy VII - Just, it made my school days bearable.
WoW - Through EU release until TBC, I will never forgot all those great times. My true introduction into online gaming. The reason so many go back to WoW is to try and experience that feeling again, for me it fails every time I try, but I keep trying. Note to self: stop trying!
Half-Life 2 - I built a computer to play this game. I followed every news story around it I could. It was released and I played it non-stop until I completed it that day. Amazing