Definitely Quake. Growing up on Wolfenstein and Doom, this game was... well, you know. And I had no 3dfx either, I was playing this on a 150MHz processor, a computer with a massive 800MB hard drive and an OTT 64MB RAM. Good days.
On that vein, FFVII - just because I was so into the SNES versions. Played V first, then an older one and bought VI on release with 2 months' pocket money. The jump in that was incredible - I nearly wet myself when Cloud drove the bike through the Shinra HQ window.