System Shock 2
You like Dead Space? You like BioShock? You like Doom 3 (god forbid)? The System Shock series is the inventor of the plot-driven horror shooter. And the second game specifically is a forgotten classic. The Cyberpunk themes mixed with horrific environments and heavy PC RPG gameplay makes it a big favorite on my list (my sig graphic is for this game, after all).
And yes, if you noticed a similarity between System Shock and BioShock's title, BioShock is basically a remake, changing the Cyberpunk elements to Steampunk and changing the setting from spaceships and space stations to an underwater city in the '50s and '60s built upon Randian principles that fell apart at the seams. BioShock isn't quite as nerve-wracking and skin-tingling creepy as System Shock 2, but it is also quite fun and is the easiest way most people can get a hold onto the feel of System Shock 2.
This game really gives you the feeling of being utterly alone, despite having help via PDA Radio that's always out of reach. And it seems to always know how to give you hope in finding more survivors, only to dash your expectations.
I've been reading different reviews from System Shock 2 and it seems to be pretty scary.