I'll go back to System Shock 2 where Glados and Atlas have their origins. Pale imitations of Shodan/Janet Polito that they are, Glados is a sweety-pie compared to Shodan, and I saw the "twist" coming a mile away in Bioshock, probably because I had played System Shock and SS2 endlessly.
Look at you hacker. A pathetic creature of meat and bone, panting and sweating as you run through my corridors. How can you challenge a perfect, immortal machine?
Edit: and not really a sudden twist but finding out the story behind The Nameless One was pretty epic.