I can't decide a single game so I'll mention more than just one.
Spear of Destiny was one of the first games I've ever played. I still love it. Oh, and I played Doom Shareware at practically the same time so yeah, it beats even Doom. Anyway, I've also learned to love Wolfenstein 3D, which Spear of Destiny is a prequel to, but SoD still wins.
Wolfenstein: Enemy Territory is probably one of the best multiplayer games I've ever played - everything worked. Enemy Territory: QUAKE Wars gets pretty close and with all these bad servers we have today (bad mods, maps, and all), it is actually better. Now: ETQW; before: W:ET; at best: W:ET.
Also, I've played Deus Ex quite a bit though I have yet to finish it. Still, I've loved it. It's unlucky I had to format my HDD when I was playing it and I've never managed to start it again :(
Then there's tons of classics I've pretty much missed: Blizzard's games (except for Diablo II) (also not sure if I'd consider them even close to 'best'), BioWare's games (and Baldur's Gate just doesn't work anymore: low non-changeable resolution + 22" widescreen = horrible), the Fallout series (and I'm excluding Fallout 3 here, though I have played it a bit) (by the way, I own Fallout 2 but have yet to start playing it), Paradox Games' strategy games (Europa Universalis, Hearts of Iron, Victoria - games like that)... And probably many more.







