The Beatles - Sgt. Pepper's Lonely Hearts Club Band
This may be the most famous album cover in history. It combined a great concept, a game and a bit of intrigue. The idea of including a collage of famous faces as if it was a group picture was a great idea. Add to that the fun of trying to identify everyone and you already have one of the best covers ever. But what really puts it over the top are its clues to the "Paul is Dead" hoax. I had a friend in high school who had a Sgt Pepper's poster hanging upside down on her wall. When I asked why it was inverted, she said, "To make you think about it." It was a good idea, because Sgt Pepper's is certainly something to think about.
The Sgt Pepper's cover has also been copied several times. Frank Zappa parodied it inside the gatefold of his We're Only In It For the Money album. The great punk compilation Burning Ambitions: A History of Punk also lampooned it.