Good (old) thread!!
For me (and many others, I suspect) it was a SERIES of albums that caused us to take left turns. It's very hard to pinpoint just one album that "changed music for you". I'd like to think that music is a lifetime journey of discovery.
But, if you put a gun to my head, and force me to pick the biggest turn/realization, I would have to say......
My discovery of Dylan's Blonde On Blonde when I was 18 years old.
My friends laughed at me when I played it for them. Fuck them!!
This album changed my world view of singing, lyrics, composition, and....yes.....popular taste. It blew my world apart. My friends had to (or maybe they never did) catch up. (We are talking 1980 here. Yes, I'm old.)