George A. Romero's Night of the Living Dead is way up there on the list, for me. Critics panned it back in '68 for it's unusually graphic nature (at the time).
That is a classic.
I'm a big fan of THE DAY THE EARTH STOOD STILL, from the great Robert Wise. I would love to riff that scene where the alien words Lincoln's words at the Memorial and remark that he wants to meet a human* like that.
*=Of course, this was in 1951, when Lincoln was practically an American God. I do miss that sentimentality of sorts, even if its more myth than reality.
