Although not a RPG, hence why I didn't vote for it, System Shock is more like a first person adventure game with shooter elements. Contrarily to Ultima Underworld, there is no leveling but the player, being gifted with a military cybernetic interface, can upgrade their hardware with various programs scattered around Citadel station. The method of movement is very similar to Ultima Underworld's, so if you're familiar with it you won't have any issues. Seeing as this is also a shooter, there is a mouselook mod available that makes the whole thing more suitable for shooting stuff and probably more bearable for people who have issues with the whole movement thing. I personally use this mod. Other domains where System Shock excels are the atmosphere, the music and, of course, its mechanics: being built on top of the already super solid Ultima Underworld games, System Shock has a great deal of emergent gameplay and simulation elements. The game is entirely configurable before a game starts, so you can setup your difficulty mode, and even set a time limit of 7 hours to thwart SHODAN's plans if you so desire. System Shock was also years ahead of its time: you could jump, crouch, lean and even lean while crouched! This was in 1994!
The atmosphere is top notch: Citadel station is a very dark place and the posh levels, mainly the executive level, are damaged and roamed by mutants and SHODAN's cyborgs. The audio logs, fully voiced in the CD-ROM version of the game, feature great voice acting and some of them are spine-chilling. SHODAN's ominous orders are very effective, and she watches you from everywhere. The music is excellent, being some techno-industrial mashup with the occasional dark ambient track, and pretty much all the themes are very memorable. The music is also dynamic, changing depending on your actions (if you are attacked, for example). But the best thing is the cyberspace to me. It's a game within a game: as soon as you enter a cyberspace terminal, you are jacked into a digital environment with absolute free movement in all 6 directions, and must destroy the defenses to open doors and achieve other objectives in a limited amount of time before SHODAN notices your intrusion and retaliates. You can also collect better softwares to help you in the cyberspace. It's a tad difficult at first, but once you master the movements, the cyberspace begins to be the best shit ever.
System Shock is a milestone in gaming and it's one of the best games I've ever played.