I am bored so I am going to post about games in alphabetical order and mention how much work/what you need to get it working the best it can.
Also I won't mention games that are on consoles unless the pc version has a leg up.
Baldurs gate 1 and 2: These games are isometric dungeons and dragons rpgs that have a lot in them, expect to put a lot of hours into these. Buy the enhanced version and look up the npc mod for baldurs gate 1, you don't really NEED mods for 2. Note theres a mod called sword coast stratagems that makes the ai smarter but thats more for a second playthrough or the crazy into dungeons and dragons type
Dark souls: This was a botched port that got fanfixed with dsfix into something better than its console counterpart, I suggest the pc versions of both souls game but 1 is clearly better as a pc game if you can throw a mod into a folder.
Deus Ex: I suggest grabbing this from good old games and installing the biomod http://deusex.wikia.com/wiki/Biomod_(unofficial_mod
). There are graphical update mods that you can find and you should install as you see fit but biomod is really the only MUST HAVE mod in my opinion.
Giants -citizens kabuto: Get it from good old games, the game is rather clunky but the humor is quite great.
The Gothic games: Only get if you are willing to put up with INSANELY clunky games. There are fan patches which are a pain to find that make it a bit better but man, you gotta work to get to the good stuff with this series. Whats up with euro devs and buggy but great games?
Icewind Dale: Same engine as baldurs gate but more combat oriented, get the enhanced edition of 1 and any copy of 2. Good games if you like your rpgs more combat than story.
Left 4 dead 1 and 2: Has more content and the games better on pc hands down. Go for this if you have friends only because playing with randoms 50 50 on if you have fun or pull your hair out. Also get a mic if you play this.
Magacite: A fun little indie game thats cheap and great fun with multiple friends, don't expect much but for the price it killed a good weekend for me and my friends
Neverwinter Nights 2: Get the good old games release and enjoy these insanely good games! there are a few mod packs that are worth getting bu t sadly the website I use to get them from is dead, another dungeons and dragons game but this one is third edition and 3d. Very fun.
Postal 2: A lot of people put down this game for just being a "Murder simulator" But the great thing about the game is you can go through it killing as many OR as LITTLE people as you want. When I beat the game I got the finishing rank of "Jesus" because I never killed anyone that didn't attack me first.
Quest for glory series: Get these from good old games, they are a mix of adventure game (Think monkey island or kings quest) and rpg (Think rpgs duh). They are my favorite game series but they are rather old and clunky. Also get the remake of 2 free online, just google it.
Sacrifice: an insanely charming real time strategy game where you take part in the battles as a general wizard, the art style is unique, the world and characters are full of charm and the voice acting is fantastic! Also the gameplays pretty fun to.
Vampire the masquerade: bloodlines: Holy god where do I start... Get the steam or good old games release then apply the fanpatch. This game has to win the award for most broken and buggy AWESOME game ever created. The people who made this game couldn't code for shit but they knew how to make a great game!
Xenonaughts: A spiritual sequal to X-com, just buy it and you are good to go.
X-com, the new one: The new x-com! Not quite the same but still good!
All the Ys games: These are enhanced remakes of the Ys console games and they made over 10 of these games for a reason, they are great. Buy them on steam