The greatest sci-fi movie in history

When I was about 13, I was sleeping over at my mate MG's house. We were flicking channels and stumbled on what was obviously the greatest sci-fi movie in history, overflowing as it was with spaceships, robots, lasers and aliens. Then his father came in and ordered us to go to bed. Fortunately, peeking out of the doorway of their guest room, I could still see and hear the TV at the end of the hall - I wouldn't have to miss the greatest sci-fi movie in history! Unfortunately, after five minutes, his dad changed the station and left it on some other program. I was devastated and, in subsequent decades, never managed to catch the amazing fillum again or even learn its name...until just recently, when I discovered what I believe to be its trailer.

Is it really the greatest sci-fi movie in history? You be the judge -

http://www.youtube.com/watch?v=pzfuNSpP0RA


[Edit: On reflection, I was probably only 11 or 12. MG and I woulda been growing apart by Year 8.]