Descent and Decent 2 pure one man ship flying, lost in the mines, robot blasting fun. Plus multiplayer. What more can you ask?
Halo and Halo 2 My favorite FPS for single player and multi player. My only complaint is co-op mode is limited to 2 players. What I would have given to play as part of a 4 Spartan team through the storyline on Legendary.
Morrowind and Oblivion
Knights of the Old Republic (2 could have been better, but it was still good)
And of course Freelancer multiplayer
There must be a bunch from the good old days of Atari 2600 and then Commodore 64 like Dig Dug, Tempest, Joust, and Pac Man but its been so long I cant remember them all specifically. Believe it or not I did have the original PONG console back in 1975 or maybe 1976 and in its time it was great.
