3. The Player & SHODAN (System Shock 2)
It may not began its life as a sequel to the original System Shock, but System Shock 2 nevertheless became one of the most influential first-person shooters ever (look to its spiritual successor, BioShock for evidence). Set on a drifting spaceship overtaken by an infectious organism, the sequel brought players face-to-face with the cyber-villain of the original adventure, a female AI dubbed SHODAN. It was SHODAN who had created the disease turning crewmen into monsters, and as is so often the case, relied on the player to help clean up her mess. Bargaining with a past nemesis is rarely wise — and almost never lasts — but the shaky alliance was a memorable one, as was the game as a whole.