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.
Yeah, playing Resident Evil for the first time was something incredibly atmospheric and special. I rented it from Blockbuster and knew straight away I had to buy it.
Before it released my friend and I used to speed-run the Resident Evil 2 demo which let you play as far as you could get into the full game, but with an 8 minute time limit.