Oy, many of your are sadly misinformed. U2 is from Ireland. Bruce Springsteen considers himself a solo performer who shares the stage with the E Street Band (they were inducted into the Rock and Roll Hall of Fame separately).
It's true that most of the greatest bands are from the British Isles; they have way out performed the US, especially by virtue of their much smaller population. The Beatles, The Rolling Stones, The Who, Led Zeppelin, The Smiths and on and on. That's what makes this question interesting.
When friends and I have pondered this one, we usually come up with CCR, The Doors, REM and The Grateful Dead. Then there are outliers, like The Velvet Underground, Patti Smith and Television. There's no clear winner, since the list depends greatly on whether record sales, longevity, influence on other bands or live performance is weighted most heavily. Overall, I usually opt for The Doors, but CCR's popularity rivaled The Beatles back in the day.