I know I jokingly said Nickelback awhile ago, but the only real answer to this question is the Beatles and it isn't close. Even diehard Stones fans who profess to hate the Beatles, do actually like them (sure they prefer the Stones, but they don't actually dislike the Beatles).
I think Elton John, the Police/Sting, and Queen are also pretty universally liked, but the Beatles is the only real answer.