The amazing thing is how Vangelis composed his electronic scores -- although he used multiple synths, it was not track by track. He would be surrounded by synths, and play everything live, one hand here, the other on a different one. Then build up the score cue by cue. Overdubs came later. No one else worked that way. Even Wendy Carlos built multitrack layers at the time, line by line using monosynths.
Speaking of good movie music, how about A Clockwork Orange?