In my opinion the Beatles are legends not just because they were some cute 'boy band' in the '60s with musical skill playing at an opportune moment in history. That's what I used to think. I was not born during the era they became famous, but when I finally gave Revolver a listen in my late teens (the early '90s) it was one of the most incredible things I have ever listened to, and I have listened to tons of incredible music.
I have immersed myself in all genres of rock, classical, jazz, pop, metal, r&b, etc., but what amazes me the most about the Beatles is their sheer passion, creativity and output. You really can't claim to have 'heard' them unless you've sat down and really listened to them, because each one of their albums sounds different than the previous one. It's rather spooky. Usually musicians are known for one style, after which their act loses momentum and gradually declines. Not so with the Beatles. Like many great artists (Bob Dylan, Picasso, The Stones, even The Beastie Boys), the Beatles go through many distinct periods and styles.
On top of that, you can tell how truly inspired they are. I think so many people keep listening to them 40 years later because it sounds timeless the way a Mozart or Beethoven sonata sounds timeless, IMO. A lot of it could have been written last week it sounds so fresh..
