A Nightmare On Elm Street ~ the tub scene....that is the scene that scared me so much that my family and I had to leave the drive-in. I will NEVER forget that....and neither will they. Both my parents still bring that up every now and again.
Oh and also "This.....is God."

The shotgun blast to the head is the first scene I think about when remembering Maniac, brilliant film.

The transformation scene in An American Werewolf in London

The end bloodbath in The Wild Bunch


A few others have mentioned these but the endings to both For a Few Dollars More & Good, Bad, Ugly are the stand out scenes for me.

The ear in the porridge in Dead Alive.