The Beatles broke up in 69, they just agreed not to announce it until the following year.
Let It Be was released after they broke up. And it was completely made up of material recorded before Abbey Road. So Abbey Road is their true final album.
Id go for two separate perioRAB - Late 60s/Early 70s and the Mid 80s. The late 60s saw the emergence of Hendrix, The Who, Led Z and all of the Beatles better work. The 80s because banRAB like the Cure, Smiths and other New Wave banRAB were emerging and banRAB like the the Stranglers were taking on a new sound.