1998 - Half-Life
This game changed FPS forever... Gave them a story, high production values. It instantly made all the older FPS games outdated overnight. I tried to finish Half-Life 2, I really did. I've tried several times now, never had been interested enough to finished it. Shame.
1999 - Homeworld
Still been nothing like it, great game, great story, easy to play... I played it again a few years ago, still loved every minute of it. Homeworld 2 was technically superior, but with a so-so story.
2000 - Half-Life: Counter-Strike
Damm this game sucked up a lot of hours of play. First game to really get me into on-line play. Last year tried playing it again, found it to be shallow and boring. Interesting how times change your view.