I remember the Beatles the first time around, and like anyone else who has listened to them, burned out on them, and then for some strange reason (curiosity, nostalgia, etc) went back to listen, think I can give a reasonable opinion why their music stands the test of the decades. The best way I can put it is primarily Paul McCartney's gift for music composition and unusual but pleasant chord changes that give Beatles tunes a sort of thumbprint of their own. That thumbprint carried through their psychedelic years despite John's constant meddling. Their music was well written, genuine, and rightly deserves a high place in pop-music history.
It was a strange time. It was an innocent period of time which later degenerated into the various experimentations of drugs and hinduism. John went the way of the marxist for everyone except himself...he was a very wealthy capitalist. Paul and George went back to the vitality of the music, though I do not believe it was ever as good as with the Beatle mix. Ringo...well what can you say about Ringo. He remains relaxed and taking it a day at a time.
I have every vinyl LP they ever put out. I still listen to them. I love the 'crackle' of the LP on the stereo. Go back and listen to them sometime when nostalgia gets under your skin.