Carmageddon demo: the "woah how can something be so violent and so fun", the quite realistic but still simple driving physics, the damage, the carnage, the freedom. most played demo, by far (probably more than the full game, definitely more than the sequels)
early 3d shooters: first it was
Quake 1 with the first real, 3dfx graphics... it was mindblowing seeing at a friends so i bought one myself just in time for
Quake 2.. then
Unreal came and yet another "woah how does this even run on my pc" (yeah, the castle flyby was awesome
), then
Q3 with the curved surfaces and volumetric fog, and then a couple of years later
Far Cry 1, when you exit that first tutorial corridor and bask in that sunlight on that sunny beach... maan, i think i was playing around on that first beach for a couple of hours
Soul Reaver (1&2&3): the story, man. that story. one "woah" moment after the other (especially in 2 and 3, even though 1 is the best game)
Tomb Raider: "woah, tits! big boobies!", yeah seeing them in a game at such a young age made an impression :D
Dark Reign: at first it seemed like a normal RTS with slightly asymmetrical races back when Red&Blue were the norm, but then i started reading the mission (de)briefings, unit descriptions.... "woah, what a story"
Total Annihilation: compared to other RTSes at the time, it was just one big "woah" moment
Blade of Darkness: oh man, dat lighting, dem levels, dem brutal secrets, dat combat, dat dismemberment/locational damage
Operation Flashpoint (demo): what the others said
Freesapce: "woah Orion destoryer" "WOAH LUCIFER";
Freespace 2: "WOAH COLOSSUS!" "WOAH SATHANAS!!" "WOAH WTF COLOSSUS VS SATHANAS" "HOLY SHIT"
Demons/Dark Souls: "woah finally a worthy successor of BoD"
Chakan: semi obscure platformer a friend found somewhere back when we were kids and played on crappy consoles... the first "woah this is face-meltingly difficult" moment (never finished the game
)
Mortal Kombat: the "woah" of the first fatalities (wears off quick)
Morrowind: the world so alien and full of wonders... (similar thing with Outcast)