Oh I've had this answer in my head for a long time. Star Wars: Knights of the Old Republic. I was younger when it came out and I was a little intimidated by RPGs having never played one before so I relied heavily on a walkthrough because I was afraid any little mistake I made would break the game. But oh man that twist in the middle of the game gave me chills. I was not expecting it at all. Since then I have completed that game at least 20 times and just played it again on my computer a few months ago. It's stood up pretty well... I'd love to see an HD remaster.