Last month, Wired published a little pie chart showing the results of a reader survey asking that very question. The favourites were:
Tetris – 23%
Other – 18%
Scrabble – 16%
Pac Man – 12%
Super Mario Bros. – 12%
Poker – 11%
Doom – 8%
Yowza, those are old school. For me, I think it’d have to be the venerable (but not quite Pac Man-venerable) Ultima 3. It’s probably my favourite because I spent more time playing it than any other game. Or maybe it’s because it was during my adolescence, and I was just discovering what games could offer. Runner-ups include Ultima 4 and The Bard’s Tale.
What’s your favourite game of all time?
DarrenBarefoot.com 
Half-Life
Have not played HL2, so no idea if I would find it better or not.
Halo for Xbox, Final Fantasy III, Civilization II, Pole Position (arcade), Galaga (arcade), Mario Kart 64, Donkey Kong Country (SNS). Ah, the memories…
Civ II and III, AoKings, Jetfighter, US Navy Fighters, Seawolf, Red Baron, Lords of the Realm…
Not quite so old-school, but pretty close…
Space Quest!
Super Mario Kart, Donkey Kong Country, and (waaaait for it) Choplifter for the Apple II plus.
Diablo II for sure (Civ II is good too)
Elevator Action!
As a kid, capture the flag. As an adult, it’s got to be one of these: scrabble, hearts, donkey kong country, or the lost city of atlantis (computer game, I think that’s what it was called). Oh and I really liked Riven and Silent Hill. I freaking hate Tetris.
Dark Castle (and the sequel, Beyond Dark Castle) on my Mac Plus! Ha. Heck.. let’s go further back: Transylvania (I, II, and III), Alpine Adventure and Cross Country Canada on the Apple //e 🙂
Julie (other commenter) reminded me of “capture the flag” which was a wide game, and we played variations of that theme on camping school trips. Some were quite elaborate with spies and secret messages as part of the scenario.
For board games I loved the game “mastermind”, I totally ruled at it.
In terms of pure nostalgic reverence, my favourite is probably Robot Odyssey I for the Apple II. I spent years of my life working my way through it.
Probably Taipan or Castle Wolfenstein/Beyond Castle Wolfenstein on the Apple ][+. Thank goodness for OSXII for Mac OS X.
Rogue
I’m glad someone said Rogue, that was one of my favourites. Aside from addictions to the usual Microsoft Office suspects, mine included the tile-matching Mahjongg, Tetris and Donkey Kong.
The Prince of Persia series for the PS2. Definitly the best three games I’ve ever played. Although the second one wasn’t all that good. The dialog of the Prince in the first game is simply awesome, though.