With regards the Paul McCartney quote, maybe also worth mentioning that it was either Lennon or Harrison that said "No Shadows... No Beatles!"
I'll leave you to decide which comment suggests the biggest (and perhaps most important) influence.
It's also certainly a fact that places like America can be incredibly polarised in terms of their acceptance of different genres of music. For example, a band like the Scissor Sisters or Sparks were probably far more popular in New York than they ever were further west. For that reason, I'd be willing to bet that (certainly at first) the Beach Boys had a far bigger following on the west coast than anywhere else. I am guessing that this probably could be verified.
Glad that I got a lively discussion going with my Beach Boys comments. It would certainly be interesting to go back in time, take them out of the equation (separately of course) and see which group did indeed have the most profound effect upon the generations of musicians that followed. Still, for the time being, that isn't going to be possible, but I'll stick with my assertion that - regardless of numbers of records sold - the Shadows made the biggest impression.