You know, that's one of the reasons I swear by the Beatles being the best artists ever. You get something great on each level; on the surface it's catchy pop with excellent melodies. Behind it lies an experimental sound and perfect harmony in each and every song. And looking even closer you discover new things on each listen, and you start to unravel complex layered sounds realizing the greatness of the music a little bit more every time.
I can listen endlessly to the Beatles, they never bore neither when played on the background, or when attentively analyzing every little sound that fills my consciousness.