System Shock. First game came out in 1994, and it was downright revolutionary by those standards: voice acting attached to a real plot (instead of some excuse plot before going and shooting lots of things) and audio logs and emails to tell the story, a highly interactive world, 3D environment with the ability to look up and down and climb ladders and stairs, lots of weapons (including multiple weapons in each category instead of a flood of very unique weapons for different situations) with alternate ammo types that you could switch between, and SHODAN as one of the most memorable villains in video gaming history. It was unfortunate enough to be released around the same time as Doom 2, which was much less complicated for the general public and was the only game that could get away with being an FPS without being called a "Doom clone" at the time, since this was before the first-person shooter became a genre of its own.
The sequel came out in 1999 and was equally revolutionary: all the stuff from the first game in a full 3D engine with even more complexity (but tweaked following the advances in the FPS genre to actually be playable with two hands), many upgrade paths, a plot that gradually unfolds over hours of gaming, a deep world that has a history and explanation of literally every item and enemy you encounter (even as simple as the history of potato chips and cola and specifics about the brand you're drinking on the ship), and a semi-open game that gives you a series of very open and maze-like decks filled with secrets and an objective to be completed however you can get to it. THIS one was killed by being released at the same time as Half-Life, and very few games could compete at that time.
Then the rights to the series and individual games got tossed all around the place, which is why Good Old Games only just got System Shock 2 available for digital download and the original game is still almost impossible to find without pirating it (to the point where the main System Shock forum includes a free digital copy of the game built by the modders to be easily emulated on a modern computer) or spending a lot of money for a rare secondhand copy and hoping that you can get it to work. Any attempts at a third game haven't gotten even slightly to a coding stage, and the developers will have to rely on the clout earned through Bioshock to try and convince their investors to give them a chance if they can even get the rights back.
