Unreal Tournament (UT99 GOTY) Great shoot'em up game to play online with diff modalities, the best one (to me) is CTF in some servers like House of Fools (hofgamingclan.com) or Pureinstagib (unrealkillers.com) but there's also Monsterhunt and Deathmatch - It doesn't need a lot of pc power and it runs great with a good gfx card.
There are a lot that come to mind, some already mentioned, like Planescape: Torment ...just an unreal game, way ahead of its time. Baldurs Gate 2 - See above The earlier Ultima series Half Life, et al Counter-Strike
I could go on and on, but honestly...I think if I really had to choose one PC game, I think the best I've ever seen would have to be Skyrim. It's just so...epic in all aspects. It's weird for me to choose that title, honestly, as I've played so many great games for about 25 years now, but bang for your buck, diversity, graphics, story, etc...it's gotta be Skyrim. I mean, even the name is badass. -- I am only one, but I am one. I cannot do everything, but I can do something. And I will not let what I cannot do interfere with what I can do.- Edward Everett Hale
Show them the PC modding power on Skyrim and they will leave their consoles.
PC Gamer did a series of articles describing what happened when they turned on 200 random Skyrim mods all at once. The title was something like Realm of the Mad God. -- Concentrated power has always been the enemy of liberty - Ronald Reagan
A game that got far less exposure than I think it deserved was one called Helbreath. That game was grindy like you wouldn't believe but the PVP made up for that 1000x. Was an MMO that used pictures to make it look 3d and the mechanics actually worked with no lag etc. So you could run it on any computer (hell you could run a decent private server on any computer too). All magic was 'skill shot' back when that concept was virtually unheard of and 1 on 1 or 10 on 10 open world duels could go on for 20 minutes if both parties were skilled enough. I would still be playing it but the official servers are dead (they might be shut down now) and the private server I used to play on is shut down.
Battleground Europe has been around for almost 12 years now and it's free-to-play. F2P allows building captures & limits certain equipment/vehicle usage but with unlimited time. There are no microtransactions, but if you decide you like the game then you can upgrade your subscription to a monthly fee. It is one persistent server and there can be hundreds and hundreds of players on at the same time from all over the world. Town battles can last hours and maps last months at a time.
Vehicles are modeled with realistic armor and penetration values and use no hitpoints.
There are also fighters, bombers and naval warfare. I know the graphics are outdated, but for those of you looking for a Player vs. Player combined Arms WWII sim, give it a try. Ive been playing it off and on for the past 10 years. Hopefully those who try it will formulate their own opinions on whether or not it's worth a download.
P.S.-Join a good active squad, it really makes the gaming experience that much better. Most squads use their own Teamspeak server or the Allied/Axis server provided by the company. S! and good luck