"Come and See" is my top pick. I think the main thing to get across to aliens is not intimate details about our culture but rather an honest look at our faults.
"Look, this movie is how we are at our worst and our best. Both sides represent some facet of humanity. But for every evil humanity wrought upon ourselves, there were people who fought against it to the death."
I don't recall the part that shows us at our best. In fact, about the only thing I can remember is the boy's old man face which is is enough to give people nightmares all by itself.