Raiders Of The Lost Ark (Steven Spielberg, 1981)
I always watched it on TV at Christmas and always saw it as a fun film. It wasn't until the last year or two that I watched it again with a cinematic eye and then started realising the genius of where Spielberg put the camera or the editing. It's quite an experimental film as well, really. There are long single takes and in-camera editing and split focus. You see it in a whole new light. That was a revelation to me. It does the joint thing of being a ripping yarn while also being incredibly well made.