FPS, Strategy and Simulator are my big three game genres.
Same here. My current favorite for each genre is:
FPS: Counter-Strike: Source (Other good ones: Half-Life 2, Battlefield Series, Team Fortress, BattleFront series)
Strategy: Star Wars: EaW: FoC (I would've put SINS, but it doesn't count because its not out yet! Other good ones: Wahammer 40k, Nexus: The Jupiter Incident, Homeworld 2, and Evil Genius - EG is funny, most people say it stinks. Sid Meier's Railroads! Demo was pretty good.)
Simulator: Rouge Squadron Series. (OGO: MS Flight Sims/ Combat Sims. I haven't tried out X yet, my computer sucks. Freespace 2 seems good, juding from this BSG mod. Star Wars: Rebel Alliance will always be special to me!)