System Shock 2. Had to stop playing before I found the first enemy from how scared I was 
Oh god, that game.
I still can hear "We Are" in my head, thinking of the enemies.
Bioshock is a watered-down System Shock 2. The former, a decent game. The latter, easily one of the top 10 games I've ever played. I'm told Bioshock 2 is just a poor rehashing of the original. Unfortunately, pretty much the only way to get SS2 these days is through pirating/abandonware, unless you trust buying overpriced used PC CD-ROMS online.
Don't forget that there's an awesome collection mod which adds tons of tweaks, fixes and up-scaled textures and sounds. It should be easily found on the web.
Bioshock was just an "elven retreat" compared to SS2. SS2 was the "fun circus".