Bionicle for the PC. When I think about it, it was really bad even for its time, but it was the first "non-educational game" I played for any computer other than Bugdom and MnMs: The Lost Formula. It had so many things I hadn't ever seen before in a game that I was just blown away; the flying in Lewa Nuva's level was just... OH MAH GOAD. And then, right after that... lava surfing. I mean, what the hell. <3
The graphics were really sh*t, the level design was downright horrible at times, the gameplay was inconsistent, and the game was chock full of bugs, but I loved it more than any other game at the time.
You know how the things you're exposed to as a child influence your tastes as an adult? If nothing else, the game had an excellent sense of atmosphere, so I may've just found the cause of why I value atmosphere in any given game so much...