This is a difficult one because the actual effect, scenery and sound arounds get better through the years. One movie that really impressed me the first time I saw it was Doctor Zivago. It was one of the first wide-screen movies and started off, before the actual story began, by showing the horror of warfare on the Russian front so simply yet so graphically: a bloodied and frozen hand sticking out of the snow; a machine gunner frozen to death and hunched over his machine gun waiting for an enemy who never came - and, of course, the haunting theme music.
It's hard to believe, but I'm told that the snow scenes were shot in Spain, and it wasn't even real snow. The story was long - it was a really long film and there weren't that many around in those days.
Yes, this would be one of my favourite movies.