Alright depends on two things:
1) How old the games you would be willing to play
2) How well your PC can handle new games
Newer stuff:
1) Crysis - not the greatest shooter out there, but nice super soldier abilities (cloaking is fun) with some fun open-world style battles against north koreans. Once you get to the second half with the big "reveal" the game becomes more linear but by that time I didn't mind because the story is actually picking up. This game is a system hog.
2) Half-Life 2 & HL2 episodes 1 & 2, portal. - these are the thinking man's FPS.
3) Oblivion - just type in google "oblivion must have mods"
http://www.uesp.net/wiki/Oblivion:Must_Have_Mods careful, you might spend more time modding this game then playing. But it's a good SP game.
4) Fallout 3 - C'mon, this shit was meant for PC.
5) Plants VS Zombies
6) Total War Medieval II - Sure TW: Empire is out, but this game can be found on the cheap and is a great strategy game with large scale real time battles. Nothing like having a couple thousand knights on the screen invading Jerusalem.
7) Bioshock
8) Company of Heroes - good single player campaign with one of those Tom Hanks style story-lines
9) World in Conflict - good single player campaign
Older stuff-
1) Baldur's Gate 2
2) Planescape Torment
3) Diablo II
4) Fallout 2
5) Deus Ex (#1)....invisible wars is alright also.
You didn't mention genre preferences. Whether you can deal with the hanky janky-ness of old games. Or how powerful your computer is.
Those three games:
1) Start with Crysis
2) Stalker - Is a little bit open world with poor in-game instructions. You lack goals until you figure out how you're supposed to play. It will play weirder than most FPS games you've done in the past. Buggy. The whole time I played I always felt like if I sneezed too hard the game wouldbreak.
3) Witcher - is point and click roleplaying with some timing-based combat. When I tried it originally they hadn't patched it and the game had 15 minute load times (no exaggeration). Make sure that game is patched or that you get the enhanced edition.