The WGA recently announced their top 101 screenplays. Check them out at the link below. I think it's a very reasonable top ten, though not in the least surprising. I don't know that there are any notable absences, but there are a few that are, maybe, not as great as the company they are keeping... I mean, Groundhog Day?
http://www.wga.org/uploadedFiles/news_and_events/101_screenplay/101press.pdf
http://www.wga.org/uploadedFiles/news_and_events/101_screenplay/101press.pdf