Aliens, not really old but I love to watch it.
Forbidden planet, a serious step up from men in crap rubber monster suits.
Metropolis, Priceless I think, for the time a really cool robot.
Dark star, Funny as hell, a bomb that argues theology hehe.
Plus an honerable mention for the Japanese animated movie Ghost in the shell, again not a real oldie but it works for me.

Cheers

Sledge