MGS2 because of how cinematic it was (the cutscenes, the sharp dialogue, the voice acting, the camera angles), the level of detail and of course the many ways you could interact with the game world.
Other games around that time period that had a similar effect on me were Metal Gear Solid 2 and Warcraft III.
Entering Phendrana Drifts, and the list goes on. My jaw dropped at the opening section and stayed open well beyond I beat the game. I knew of the franchise but I never played a Metroid game before Prime.
I was completely floored by the game and just had to get it at launch (at that point I only got games during christmas or my birthday).
I had never seen anything like it, and the game turned out to be Metroid Prime. There was a sizeable crowd of people that surrounded a booth in a videogame store and I walked in there to see what the fuss was all about.
