Ocarina of Time (N64) - I know lots of people have said it, but it still genuinely is the greatest thing i have ever played. I thought everything about it was perfect.
Burnout Revenge (Xbox) - I know alot of people didnt like this as much as the others, but this was the most fun i had with a game in a very very long time. I spent so much time with this, and still do.
Halo:CE (XBox) - Xbox, Day 1, Jaw open, much fun, blood gulch. That is all.
PGR2 (Xbox) - Almost purely for the amount of fun you could have online with it. Best online experiences of my life. And powersliding that Enzo round Edinburgh was amazing.
MGS (PS1) - Played this not knowing what to expect (I was an N64 fanboy, and no interweb) and was promptly blown away. I remember thinking WOOOAH when my footprints were found :lol Loved finishing that game.
Sonic the Hedgehog (Master System) - I know the Megadrive/Genesis version was better in almost every way, but there is still something about this version that i love to pieces. The waterfall in the jungle and Scrap Brain zone were my favourites.
Perfect Dark (N64) - Yeah Goldeneye was omgwtf amazing, but even though the framerate got pants, this is one of the few games that - for me - exceeded the uber hype. Still boot it up to fly through single player. Multi at the time was unbeatable.
Geometry Wars (360) - Yes there are other games on the 360 i would be quite happy to put here (Gears, PGR3), but GW is so stunningly simple, yet well done. Custom soundtracks and playing it over and over never gets old.