Music

I know that I own lots of music. I also know that there are enormous holes in my music education.

In your opinion what one album should I listen to/own?

(If there are more than one and you cannot choose then two would be OK, but I would like to know what you see as the pinacle of essential listening!)

You may well be surprised by the gaps in my knowledge, so please do not be put off by thinking I already know/have it!