For me it was...
Thief 2 (pc) - the story, presentation and even voice acting totally wowed me. The graphics style is the coolest I have ever seen in a game. And of course the use of sound as the most important gameplay element has not been improved upon yet, not even by Thief 3.
Chrono Trigger (Snes/PS1/DS) - the story, the music, the neat graphical style (including the beautiful cutscenes in the PS1 and DS versions), it all makes Chrono Trigger one of the finest RPGs available.
Super Monaco GP (Amiga). No kidding, this game really wowed me when I was a kid. To me it was the most awesome racer available. That was until I got Lotus 2, Supercars 2...
Morrowind (PC). The first time you get off the boat and the rich and glorious fantasy world is presented to your eyes... I just stood there, looked around and went WOW. After that I hastily went through the character creation process and I went for a long walk, and I kept walking and swimming until I had visited and seen most of the major towns in the game on foot.
Ishar 3 (Amiga / PC). I have to admit, graphically this game is a pearl and it has a very rich atmosphere. It is only too bad it is damn near impossible to complete without a guide. Still the first time I started to walk around the city and got beat up by bandits I went WOW. When I went into a store and the music started to play I went WOW. When I went through the first portal and got drawn into the forest (with amazing graphics and sounds), I went WOW.
Final Fantasy 7 - what can I say, the only game ever that made me shed a tear. The *SPOILER* scene at the end of Disc 1 where Aeris is killed by Sephiroth is the most emotionally impressive scene I have seen ever in a game. A big WOW for that. Also the impressive materia system wowed me completely. the insane combos you could make were vast and endless.