For me it's Metal Gear Solid (PS1). Up until then you could probably classify me as a "casual" (I know, terrible). But MGS changed all that. It was 1998 and I just remember having never played a game quite like it. The story, the characters, the gameplay, everything was just amazing. Back then games weren't hyped to hell like they are now. And I also didn't spend my time spending hours looking into previews or reading reviews. I just picked up a copy and had zero expectations. Luckily, I was in for a treat. How about you, SW? What game got you into gaming?
PS 1998 is imo the best year in gaming. Half-Life, MGS, Ocarina of Time, Starcraft - holy cow!