How about Kansas? I know that they get dumped in the corporate rock category, but just listen to stuff like Song for America, Cheyenne Anthem, or Miracles out of Nowhere and that is pure genius with a lot of purely American references.
So many great bands have been named, but if the question boils down to who do I listen to the most? It's no contest - Tom Petty & The Heartbreakers.
If like Kansas, rock on, but they don't belong in any but the loosest of "best American bands" threads. "Carry On..." did kinda rock, though!
To answer this question you would probably want to define "Rock N Roll". Are the Beach Boys more "Rock N Roll" than a ZZ Top or Skynyrd? For the sake of the argument and or question I think the following would be great choices. Johnny Winter and.. Lynyrd Skynyrd ZZ Top KISS Van Halen GNR Cactus Foo Fighters