Difficult question, but "Tarkus" by ELP was a turning point, up until then I had mostly listened to Motown, a bit of pop or a bit of reggae. When a friend played me Tarkus I was blown away, I went home and dragged my sisters old Bontempi chord organ out of the roof space and started learning chords and scales. I also started playing around with simple transistor oscillators tying (without much success) to emulate a synth.
With Tarkus a door was opened which lead back to “In the Court of the Crimson King” and pulled in Yes, Genesis, Pink Floyd, Strawbs and many, many more that would become a big part of my musical life.