I have been an Alien and Predator fanatic since I was about 8 years old. I still remember how I got into the films (the details I won't bore you with). But when this game came out, our family had just got our first really good computer at the time (a whopping 6GB hard drive at the time. Which makes me laugh, seeing as how I have more than that on my PHONE now). I had just started getting into computer games, with Unreal Tournament, Counter-Strike, Half-Life, nerdy stuff like that. Anyways, it was 1999 when this game was released, I was about 13 when it was released; and wow. I was in a state of awe, the graphics were phenomenal for it's time (still tolerable today) and never had I seen any video game capture the atmosphere of a film so well. It feels like you are actually in the movie. While playing as a marine; the flashing emergency lights in an otherwise dark environment, not sure if you see what you think you see in the distance, and before you can blink there's an alien charging at you! As a Predator, cycling vision modes with that heart-beat sound. Being cloaked, looking down on unsuspecting marines before you vaporize their torsos. Last but not least; the Alien. Lightning fast, vicious tail and claws, scale any surface, drop from any distance, see in any spectrum of light. This game nailed everything! The only complaint is that the predator's arsenal was fairly lacking, so it gets boring to play as one fairly fast. So, after all this time and it's aging graphics, it's still the first game I load onto my computer when I reformat or buy a new computer, it's always good and I never get sick of it. AvP 2 was fun and all, but it seemed a cheap arcade version of the first.
P.S. The online was also incomparable for it's time, the game modes, the levels, the overall fun of it all. It'd probably be near impossible to find any servers to play on nowadays, but if you can convince a friend to play this, the co-op mode is lots of fun!