Sooo, 1999 we're here !
Let's go for the nominations...
Silent Hill : one of the hallmark in horror games, and one of the few with a really disturbing atmosphere. No sure it aged well, but it definitely left its mark.
X-Wing Alliance : not as good as Tie-Fighter, but then I'm partial to the Empire so it may explain that. One of the few last good space simulator though, with a decent single-player campaign.
Freespace 2 : the other good space simulator or the twilight era.
Unreal Tournament : I never liked Quake 3, which was pretty bland in my taste. UT, though, managed somehow to take a MP game and make it interesting both in solo and multi, largely thanks to an excellent bot-system with very "lifelike" IA. I still kick it from time to time in to get some action fun.
Final Fantasy VIII : one of the most polarizing FF, half of the FF fanbase consider it the worse, the other consider it the best. I'm from the latter part, and am particularly attached to the characters, despite some flaws in the story and gameplay.
EverQuest : not really my cup of tea, but it can't be denied that the game had an enormous amount of influence, especially on its even-more influencial successor WoW. Defined the genre for more than half a decade, most other MMORPG being mere copycat.
System Shock 2 : immersive, deep and intelligent, a great game sadly often underrated (or downright unknown).
Homeworld : one of the most beautiful, immersive game, and one of the very few (only ?) space strategy game allowing to see the thrill of huge battleship decking it out. Very contemplative and pleasantly slow-paced, allowing to sink in your chair and enjoy the atmosphere. A singular, original and one-of-a-kind game.
Alpha Centauri : can't believe nobody nominated it. HELLO are we in Civfanatics forums or what ? Decidedly far in advance of its time, this is one of the best game in the Civ serie, with many groundbreaking advances that would influence heavily the subsequent Civ3 and 4. Complex and detailed, with an amazingly creative and still realistic take on the future, it's one of the very best game in the entire genre.
Planescape Torment : Looking for an incredibly well-written, deep, profound, immersive and twisted game throwing all the clichés out of the window and putting the fantasy world on its head ? Look no further. Underrated at its release, but became a legendary cult game in the following years (most asked for game on GOG, and sold out the promotional copies in a mere four minutes), the masterpiece which gave Chris Avellone his worship-like statut.
And my vote is easily :
PLANESCAPE : TORMENT
Let's be clear, Torment is not just exceptionnal, it's simply the BEST GAME EVER, bar none, and I'm not using it for emphasis but simply as a firmly-held opinion I've had since I played it. Half-novel, half-game, 100 % awesome, the only downside I could find is just how any game you play after it seems crude, dumb and superficial in comparison.