The Beatles - The Beatles (aka White Album)
To my knowledge, this was the first rock cover that dared to be this simple. By having the vision and courage to strip the cover down to nothing*, The Beatles established a standard that no one has surpassed (and very few have even tried). It's proof that sometimes the most compelling design is the simplest one.
The original pressing was a glossy white cover designed by British artist Richard Hamilton. The band's name was embossed (not printed) at a slight angle, and a unique number was embossed on each cover.** As happened with The Wall, subsequent editions had the words "The Beatles" printed in grey or black, a change that destroyed the integrity of the design.
* Part of this cover's beauty lies in the fact that it can be interpreted in so many ways. Since white is the inclusion of every color in the spectrum, I see this album cover as The Beatles' statement that they included every color of their musical spectrum in this recording. In that sense, the cover doesn't represent 'nothing;' it represents 'everything.'
** To me, giving each album a unique number recognizes the unique relationship that each fan has with an album. By doing this, The Beatles acknowledged the most important color of their musical spectrum: each and every one of their listeners.