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.
Oh boy I have a few:
PC
Another Lifeless Planet (and me with no beer) was fantastic for a text adventure.
Testdrive pushed graphics hard
Wolfenstein 3D was incredible at the time being surpassed by Doom then Quake.
Day of the Tentacle for its high quality cartoon animation opening sequence.
Unreal on a Voodoo graphics card was something else.
HL2 of course with its physics (ragdoll) engine and jump in polygons.
Doom 3 for its advance in polygon count again.
NES
Super Mario 3 was a leap believe it or not. Blew people away back then.
Battletoads had huge sprites which wasn’t a NES thing until they did it.
SNES
Fzero and Mode-7 graphics
Donkey Kong Country, its CG was nuts at the time
FFVI’s snowfield theatrics
Starfox
Sega
Earthworm Jim had a great art style and pushed edgy games
Another World/Out of this World for that opening and style was amazing and still holds up!
N64
Super Mario 64 Japanese demo at Babbages in a mall before the US demos came out. That was mind blowingly smooth.
Waverace 64 for its water effects.
Ocarina of Time for its cinematic 3D story telling and fun gameplay.
Neogeo
Metal Slug was incredible in the arcade for all the sprites moving on screen and action. So much fun still to this day.
PlayStation
Resident Evil was incredible for the atmosphere.
Tomb Raider for more detailed 3D environments.
Warhawk also stood out for its great use of the analog dual joysticks.
FFVII for its cinematic story telling and FMVs.
Wipeout for its fast paced racing and great OST.
Grand Turismo for its photo realism simulation and physics.
Dreamcast
Seeing the demo of Sonic Adventure blew my mind since it was so fast and colorful. The whale jumping after Sonic as you raced away was burned into my brain.
Gamecube
Wind Waker, still love that art style and loved the exploration that was enabled by sailing the sea. Didn’t feel that again till BoTW.
Wii
Super Mario Galaxy for its outstanding OST and gameplay.
Playstation 2
Grand Turismo 3
Little Big Planet had incredible art direction and unique play style
Wii U/Switch
Breath of the Wild. What an intro to a new world! They absolutely pushed the hardware to the limits on the Wii U.