This is a toughie, and something I often puzzle over. So many great bands were British, and this REALLY highlights that.
But I say Byrds, REM, or Ramones. The Byrds invented practically EVERYTHING: folk-rock, jangle-rock, psychedelia, space-rock, country-rock, California-rock. Their sound was practically untouchable, too. I would call them the American Beatles. R.E.M. took leaves from the Byrds book and made them cryptic and intelligent, pretty much inventing alternative rock, a far-reaching and dominating genre. They might be the American Smiths. Finally, the Ramones took power-pop and glam-rock to its logical conclusion, inventing punk-rock and really touching the soul of rock 'n roll. (Nowhere near as stupid as some of the punk bands who followed, either.)
Any thoughts on the matter? I hope so.