System Shock 2 reached a level of immersion and involvement with your environment that has never been matched before or since. Some people might disqualify it from the genre of strict FPS, since the action is less frantic and has a more cerebral slant than games in the traditions of id or Valve or Bungie or Epic, but if the first person perspective's great strength is its ability to transport the player into the role of the hero, to put you in their place, then no game does this better than SS2.
The ground breaking sound design plays a big part in this, as well as the carefully paced moments of intense emotion that the game seems to be able to turn on and off like a tap. The timing of events and engineering of your every situation is so well planned, so carefully arranged that your natural explorations and progression through the haunted ship advance the narrative in a totally transparent manor. Playing it feels like you've been cast in an excellent horror film where each scene has a specific purpose and the result is so engaging that you find yourself unable to stop playing. That's why System Shock 2 was and remains the most progressive, ground breaking title in the FPS genre.
-Tom Murray, Spartek Systems
System Shock 1 was probably a bigger leap, but in System Shock 2 the elements were more smootly entwined. It had a good story and narrative to drive you by, it had multiple possible tactics (PSI were such a change of palette) and it had the right atmosphere to it. And the minigames, I loved them. Notably, up to date SS2 is still one of my favourite FPS, topped only by F.E.A.R.