Unreal (yes the original) when I 1st installed my GeforceMX and saw lighting and mist in game.
MOHAA Omaha Beach. It was basically like being in Saving Private Ryan.
As if I forgot about that. It was superb and in general MOHAA would be the first game to completely WOW me. Up until then I was a console player and went round a mates on dinner at college and he had 2 high end rigs in his room both running MOHAA. I tried it online and never went back college that day.
Not long after my Dad bought a PC and I spent so many hours playing it.
Pretty much this when after I first installed my voodoo card WAYYYYYYY back. The opening cinematic
Another one would be Everquest. The first time I entered East Commons (the auction house) and realised just how big the community to this game was.
Have some memories:
http://www.frankklepacki.com/portfolio/game-CnC.html
Still one of the best tech demos of all time..
COD MW2 -When you get petrol poured on you and then Shepherd throws his cigar on you and you die; I was expecting to be saved. Kept thinking that Price was going to come in all guns blazing.