Matewis: please use your own definition of old or classic :)
Speaking for myself the following comes to mind :
ET
The Edge Citizen Kane
Back to the Future 2 an 3
All of the spaghetti westerns
Unfortunately the internet has has graciously supplied me with significant spoilers to the first three. The first one I don't mind so much, but I really would've loved to see the 2nd and 3d one without spoilers :(
How about you?
edit : oh yes and I've definitely got to add
Ben Hur
Lawrence of Arabia
Watch Citizen Kane with a view to filmmaking, not plot; 'cuz it's kinda all over the map. Think about directing and camerawork and how most of this stuff had never been done before.
Then... immediately after, watch "The Battle over Citizen Kane". To understand what the heck the main plot was about (other than, you know, "Rosebud").