The most unique album cover I’ve seen is the original Sticky Fingers with a real functioning zipper on the front. Andy Warhol (a friend of Jagger) took the cover photo.
Roger Dean’s artwork on Yes LPs was great, especially when accompanied with hallucinogens of the day while listening to the music.
For pure cheese, Raspberries released an LP with a "Scratch & Sniff" sticker that smelled like...wait for it.... Raspberries!