Okay, I did it-
IV and Black Dog.
I borrowed this kid's cassette at Scout Camp in 1987. I'd never heard Zep before. I thought it was amazing. A lot of bands were recycling their material in '87, but I'd never heard anything quite like it. I'd heard the Who and the Stones, but Zep was a lot more all over the place with their music.
Anyhow, IV will always stick out in my mind, and the opening track blew my mind. I rushed home and bought it!