Tetris and GTA: Vice City

Based on my stickers, posters, pillows and clothing I have to say Grim Fandango and first Mass Effect.

Quoting: SlackdogElite - showing my age there :P

I'd forgotten Elite; when I wasn't playing M.U.L.E. for the umpteenth time, I logged many, many hours on my C=64 trying to get the next "Right On Commander!"

And eventually getting it.

there have been so many over the years
C=64 I would have to decide between UltimaIV , Turrican and Soul Crystal
PC i loved Privateer/Strike Commander, Planescape Torment and The Longest Journey (funcom has always had some brilliant storytellers), Doom 1.
Then there is the whole MMO genre where every major patch has a tendency to change a game from brilliant to utter garbage or vice versa.

But if I look into somewhat more recent games then I'd say the most outstanding one that keept me facinated from beginning till the end was Shadowrun: Dragonfall

The best game(s) that I have ever played, and still play are the original three Ratchet and Clanks. Just started another play through of R&C1 on my brothers' PS3 and waiting on the new R&C (please don't be crap!)

I just wish there were more 3d action platformers, especially for Linux :S:

starcraft series ,
guild wars series,
Tera Online (just for its graphics and machanics. )
Doom series.
Dead Space Series
Crysis Series.

Planescape: Torment, Silent Hill, Il2Sturmovik - ex aequo
Legacy Of Kain: Blood Omen, SR1, 2, Defiance (maybe even with above)
Syberia

UFO: Enemy Unknkown, TFTD
Hired Guns (both on Amiga)

But now, playing games on Linux, i've discovered many beautiful indie games and I cant call them any worse than old ones. Some of them makes me feel like back in 1999/2000 ;)
Transistor (masterpiece imho),
Shadowrun: Returns (reminds me somehow Perihelion - old Amiga title too)
Steamworld Dig
Halfway (like old, good UFO)
Victor Vran

X3 series and X: Rebirth (Rebirth is really good now, after some patches) - better than Elite: Dangerous but without multiplayer. I can't forget three days of trying to capture M3 class Mamba cause of f...n proximity mines :)