Best game by the most amount of time sucked into it:
Sword of the Stars: Wow, there's ten million years of replayability in that one.
Titan Quest (plus Immortal Throne): I never thought I'd play a hack 'n slash this much, but they sure got everything right. Shame that THQ had to quit supporting it. I'll probably still be playing it when D3 comes out.
Medieval 2 (for the mods, although M1 and Rome had some pretty awesome ones too). That Third Age mod is the schizz.
Return to Castle Wolfenstein (released late november 2001, so only by 10 years, 5 weeks, including it!). Best FPS in a long time. You can keep your Call of Duties and Battlefields, just give me this for me and my friends for the rest of my life.
Best game by "Holy crap, where was this game all my life?"
Trine. Damn if I haven't seen a good platformer with great puzzles and mechanics in the last 10 years.
Mount and Blade (plus Warbands, I only started playing the first one a couple weeks before warbands came out). Wow, the initial gameplay felt so fresh, unique and awesome.
Portal. 'Nuff said, puzzles are awesome.