Deus Ex, System Shock 2, Vampire The Masquerade Bloodlines, all of these still hold up, and are totally worth a play even if you never played them back in their day.
Also, Alpha Centauri has SUCH a great narrative. Each faction has a strong identity, each leader has a fitting personality, the whole package is great.
It really deserves a remake to update the controls and UI, it still plays really well if you can get past that though.
The Internet’s Own Boy. Highly recommend it, someone else posted a comment with a link to watch it:
It was playable just fine on the PC. The console versions were the ones that really suffered
God I would give so much to experience Splinter Cell 3’s multiplayer again. It was perfect. An improvement upon the already excellent MP of Pandora Tomorrow, but still within the spirit of the game.
And then they fucked it up trying to make it more like COD or something.