The number of "firsts" that the Beatles accomplished (or, "fast follower" seconds, where they took a very little known idea and made it mainstream, like lyrics on a record. Zappa did it first, but after the Beatles, EVERYONE did it.) Listen to the snippet by Billy Joel on the bonus disk to the Greatest Hits box set, where he talks about being part of a nation that was stunned into silence by the assassination of John Kennedy in November of '63, and then not even two months later, we get these four aliens from Britain wanting to "hold our hand", and month after that, a THIRD OF OUR COUNTRY watched them sing "All My Loving", something else, and "Please Please Me" on television (virtually unheard of then for a pop group). They changed lives.
I love that line from John, when he was told that "no one would play a single that is seven minutes long" ("Hey Jude", which was 7:11 long): "They will if it's us." And they did; it spent something like nine weeks at number one.
One could make these arguments about Zeppelin, as well.