Ten is tough off the cuff. A few that I can't imagine not including:

The Bicycle Thief, Battleship Potemkin (visually mesmerizing), Cinema Paradiso, among foreign films. At the same time, I would include films that the snobbiest film critics might sneer at for being too commercial, but I think are great, like North by Northwest (a perfect movie) and Jaws (still Spielberg's best film). It's now fashionable to label Citizen Kane "boring" and "overrated," but technically, it has its place near the top. Eastwood became one of our greatest directors and his greatest work is Unforgiven, which I would place near the top.

I'd need more time to whittle it down to ten.