For me, they are:
Titus Andronicus: When Titus (Anthony Hopkins) makes a feast for his enemies and feeds them food cooked with the blood of their sons (he did that for the revenge of MANY MANY awful things they've done to him, his sons, and his daughter).
American History X: When Edward Norton's brother gets killed.
Fight Club: When Edward Norton realizes it all!.
The Game: The whole movie is a

! but the end when things start to unfold is awesome.
The Others: When Nicole Kidman realizes that THEY are actually "the others" !
Thanks for the brilliant topic Ghayath