[LintMan]

RTS: Total Annihilation, no other RTS else comes close.

TBS: Conquered Kingdoms (by dearly departed QQP)-old school, lousy EGA graphics and ridiculous music, but great AI and a wonderfully balanced set of units.

FPS (single player): I'll pick 3: 1) Half Life - great story, groundbreaking technology for its time. Most FPS's today still don't match its single-player quality.
2) Medal of Honor: Allied Assault - much of the game was standard fare, but the Normandy beach level in particular is awesome.

FPS (Online): Half Life Team Fortress - Awesome team-based play with specialized classes. I was in a very strong clan competing in several leagues and this game sucked countless hours of my time.

RPG: I've been a fantasy RPG player back from the Temple of Apshai days on my C-64, but I'll say Knights of the Old Republic is the best RPG I've ever played.

4X: MOO2. Shrapnel's SEIV comes closes but is missing...something. MOO3 was a piece of garbage that I returned as soon as I discovered that the game was unable to save your turn information mid-turn.

Arcade strategy: Archon, Archon 2: Adept - yeah, I loved them both!

Action Adventure: Star Control 2 - probably my vote for greatest game of all time. Funny, fantastic story, great music, fun action combat.