Okay, this may sound weird, but I;m opting for Demon's Souls here. Specifically, the Dragon God Boss fight.
Of all the commercials that got me hyped, it was the one with the Dragon God that really made me want to play it.
Then, after playing the first little bit, I notice how the bosses generally get harder and harder. So after managing to somehow survive Flame Lurker(without magic) and learning the Dragon God was next, I sorta shit myself. 20 levels later when I finally worked up the courage to take him on.......I felt like I was just rick-rolled by some dick game developer. i mean, really? 2hits? 2-fucking hits and he's down for the count? If nothing else, Demon's Souls managed to piss me off as it always had, though in the exact opposite way...