I remember a few from various stages of my life (born 1984).
Seeing the demo footage of Sonic 2 in Woolworths and thinking the leaves falling down in Aquatic Ruin zone was so cool and advanced.
The original Sega arcade of Virtua Racing with the moving cars completely blew me away.
I remember my uncle loading up Cannon Fodder on his Amiga, and a REAL song with REAL music came out, along with REAL photos. I was amazed haha.
A few years on I remember a PlayStation demo disc having promo footage of the first Gran Turismo and it looked so real to me, I watched it over and over. The first Driver on PS1 looked absolutely amazing to me also.
Born in 1980. Seeing the original Mortal Kombat arcade for the first time at a smoke-filled bowling alley that when I was in 7th grade was pretty awe inspiring.
Aside from that playing Wolfenstein 3d for the first time was really trippy.
Yeah, I very nearly added seeing the Mortal Kombat arcade to my original post but decided maybe I was writing too much!