Original: System Shock
In case you're not familiar, System Shock 2 was a 1999 PC shooter from Ken Levine and co. and Irrational Games, the makers of BioShock. It's considered to be a spiritual predecessor to the studio's later series, and it's where many of the great ideas in BioShock originated.
Irrational and Looking Glass Studios were originally creating a brand new game, but when EA signed on as publisher they altered the story to make it a sequel to the original System Shock. Naturally, it completely transformed the series, since it was never meant to be a System Shock title in the first place. But it was definitely for the best.
Some things were similar between the two, like the combination of shooting, role-playing, and horror elements. But with state-of-the-art technology and Irrational's unique touch, System Shock 2 was a huge improvement.