I had to vote for Angel's death. I mean, the first time you see it you've got to know that she's going to die, but I, at least, had some little hope in the back of my mind that ole Thrakky would keep her alive to bait Blair with. By the time I find out in character, though, I'm so shocked because I've gotten so into the game by this point. No matter how many times I play it, I'm still surprised.
Hobbes being a personality overlay came a really, really close second. I always flew with him as my wingman, always. He was my absolute favorite to fly with 'cause I trusted him and knew he could get the job done. Even before I read the book, I thought the spy was someone else, probably Cobra. When he defected, it surprised the hell out of me. Personally, I always chased him down and blasted him straight to hell. I remember thinking when I saw Cobra bleeding all over my hangar deck: "Well... That rules her out."