Alien 3. The movie has it's problems, but that ending it THE ending of trilogy endings. Ripley, knowing she'll never find happiness, and in a last act of defiance, sacrifices herself to bring the end to the xenomorphs for good. It just feels so finite. The triumphant music, the prison closing down for good, and Ripley's original transmission FINALLY reaching a receiver, only so many many decades too late.
Also, the movie was ass, but the ending to Terminator 3 is so poetic. The violin music with John's VO narration, it gives me chills every time.
"There was no system core; it could not be shutdown. The attack began at 6:18 PM, just as he said it would. Judgment Day, the day the human race was almost destroyed by the weapons they'd built to protect themselves."