I have a very long list of games...
PC
Knights of the Old Republic I & II: I recomend buying both games as a bundle, aside from the game play (which is exellent) there is a wonderful story that plays out through both games. The Star Wars Universe is wonderful in this game and almost everything is accurate to the movies and (or) the books. The only complaint I have is the some of the strange and sporadic bugs that pop up now and then. (most of which were fixed with updates)
Also, there are some great mods, skins, and items that can be downloaded that add alot to the game play.
FINAL RATING: 7/10 (great!)
Half-life 1 & 2: whoaaa its like the perfect sci-fi! aliens, guns, girls... yup its got it all... but there are some downsides... The AI for one isan't perfect (in the second game), I would have thought that after so many years of developement they could have done better. Also there is the little matter or STEAM >_<!
Steam is the most horrible thing to ever grace this planet... You have to authenticate your game through the net everytime you play. Bah! I hate it. buuut the games are so great that this can almost be overlooked when you start playing.
FINAL RATING: 9/10 (wonderful!)
these are my favorite single player PC games but there are many more I'm not giving a good short review for... so I'll list them now :P
F.E.A.R (FPS)
Far Cry (FPS)
The Elder scrolls III: Morrorwinds (FPS/RPG)
Gothic 2 (RPG)
Unreal Tournament 2004 (FPS)
Neverwinter Nights (FPS)
Battlefield 2 (FPS)
Tron 2.0 (yes its actully a good game, FPS)
Dungeon Seige (RPG)
Splinter Cell Series (3rd person/stealth/action)
Mechwarrior Series (FPS/RTS/RPG)
GUN (3rd person)
Doom 3 (not as good as I had hoped but still a great game, FPS)
Call of Duty I & II (FPS)
Final Fantasy VII (RPG)
Homeworld (RTS)
there are alot more I'm not thinking at the moment.