The first PC games that I ever played were Battle Chess & Maniac Mansion. These were both DOS based games in the late 1980's. I have been a PC gamer & enthusiast ever since that point. Building my own PC's since my Slot 1 Pentium II & made a career out of my passion. So you can trust that I have some pretty good knowledge on the subject . I will give you a great list of 25 games of my personal favorite single player PC games that should keep you busy for a very long time. Most of these games can be purchased for fairly cheap off of Steam or Good Old Games. Some you may not be able to find outside of ebay or other means. I'll leave that to you to figure out . Keep an open mind about older games as I know some may have not aged well. These games were epic for their time & obviously, the flare & presentation of modern games will greatly outshine some of the games on I will list. These games are in no particular order.

1) Deus Ex - Still the best Deus Ex game in the franchise. Deus Ex Human Revolution is an excellent game as well. Deus Ex is one of my favorite games of all time!
2) System Shock - Played Bioshock? System Shock started it all!
3) Gothic ( Franchise ) - The older games will be a pain to play now but are worth the experience.
4) Elder Scrolls III - Morrowind - This is a much better game than Skyrim is in my opinion. This may not be the case for those that have never played it when it was a new game.
5) Half Life/Half Life 2 - This franchise of games are must owns.
6) Grand Theft Auto IV - Great game in its own right but the mods bring this game to life. Very high end system recommend for the graphical overhaul mods.
7) Mass Effect ( Franchise ) - Three games you simply must own.
8) Freedom Force - Still one of the best, & funniest, super hero inspired games to date.
9) Fallout 3 - An excellent game. I would expect that you've played this already. Fallout New Vegas is a few steps in the wrong direction but still a decent game.
10) Fallout 1 & 2 - They are much different games than Fallout 3 was but they are amazing games none the less.
11) Portal ( Franchise ) - Excellent games with tons of charm & personality to them. An gamig experience that you wont forget.
12) The Witcher ( Franchise ) - An action RPG franchise that holds nothing back. It's dark, gruesome & a hell of a good time.
13) Bully - Think Grand Theft Auto without the cars & open world. Another great game that will make you laugh plenty.
14) No one lives forever ( Franchise ) - Two games you just need to play. An excellent spy themed FPS with a very high comedic value.
15) F.E.A.R - Great FPS game with one of the best AI's to this day. The first game was great. The other games in the franchise were pretty decent but the first game is a must own.
16) Batman Arkham ( Franchise ) Two games that are definitely worth playing. A third game is in the works.
17) Bioshock ( Franchise ) - Bioshock 2 was meh but the original & Bioshock infinite are great!
18) Star Wars - Jedi Knight ( Franchise ) Some of the best Star War based games ever made. They still hold up very well on the PC.
19) Tropico - Franchise - One of the better & more balanced simulation franchises. It has a little bit of everything while not taking itself to seriously. Still offers plenty of challenge.
20) The Sims ( Franchise ) - It's the Sims....... yeah...
21) Splinter Cell ( Franchise ) - My favorite stealth franchise. Yes, I like it more than the Metal Gear franchise.
22) Amnesica: The Dark Descent - One hell of an experience. If you're easily scared or jumpy, I wouldn't recommend this game! :P
23) Indigo Prophecy - You play this for the story that will suck you right in. This is more of an experience than a game.
24) MDK!!!!! - No words can express how excellent this game is. You just need to play it
25) MYST - This will be boring to most these days but it's still one of the very best point & click games still.

5 Games with strong multiplayer aspects - These games are great single player experiences but have a strong multiplayer aspect to them.

1) Starcraft ( Franchise ) - Nothing needs to be said about this franchise. Just play it!
2) Rainbow Six Vegas ( Franchise ) - I still consider these to be the best Rainbow Six games ever released.
3) Call of Duty ( Franchise ) - Yes, the multiplayer gets crapped on by a lot of people, including myself. The single player experience, while short, should not be look passed. Most of them are very well done.
4) Battlefield Bad Company (Franchise) - Admittedly, I've always enjoyed BF games for there multiplayer aspect but the Bad Company single player experience is excellent.
5) Diablo ( Franchise ) - While the game is playable alone, this is a franchise that is much more enjoyable when played with friends.

I could throw another 25 games at you but this list should hold you over for awhile. If you want more game listed, just alert me & I'll add another 25 . Not to shabby for my first post.

-LatinLegacy