Depends on your criteria definition of "Best" and "Rock". The Beatles weren't any more 'rock' than The Beach Boys or Simon and Garfunkel. And as notable as The Invasion was in a short period, America still had CCR, The Byrd's, Sly Stone, The Doors, Jefferson Airplane, Frank Zappa, The Grateful Dead, Buffalo Springfield, Santana, Allman Brothers, all from that same early era.