Gunship on Amiga - The first proper simulator I saw. I was way too young at about 8 or 9 but I loved watching my dad play it
Shadow of the beast - Music and graphics were incredible for the time.
Stunt car racer on Amiga - The first 3D driving game I played
Doom - The first time I saw a demo of Doom on my Aunties 386 I almost jizzed. Getting the supershot gun was also special, especially the sound
Donkey Kong Country on SNES - This was way ahead of its time for graphics and everything really
Crash Bandecoot demo on PS1 in the shop - It ran so smooth, looked great and was very fun
Virtua fighter - The first 3D fighting game I played
Unreal 1 on 3DFX - This wasnt a my goodness moment it was jaw dropping moment. It was way way ahead of its time. Reflections, smoke, the huge outside areas, water effects...
Need for Speed 2 SE on 3DFX - IMO still the best of the series. Great graphics, fast and fun
Half Life - The first game that had a 'proper' storyline
Farcry Demo - The islands, water, sunsets etc looked amazing
Crysis - Similar to same feeling as playing Farcry
The one that has made me go "my goodness" the most has been Unreal, closely followed by Doom