System Shock 2. Spoiler tags because I'm betting none of you have played it. Some of you may not have even been born yet...
That blew me away.
At last, someone knows the Custom Robo series. And indeed, Custom Robo would be the game I'd select for most shocking plot twist.
Atlas, the guy who had guided you from the start turning out to be the bad guy (Fontaine) and using you to get control of Rapture... also the fact that you were brainwashed to do any command as long as they said "Would you kindly"...
played system shock 2 a very long long time ago. its one of the best games i've ever played...its also the only game i could never finish who can ever forget Shodan that was one truly epic game.
Oh I really can't believe myself over forgetting that. It indeed was a classic twist of story.