I know they've already been mentioned, but I think at the end of the day I might go with Creedence Clearwater Revival.
But I'd like to mention two dark horse candidates as well, due to their influence (both bands spawned or directly influenced several whole genres), even though they weren't "popular" when they were around:
The Velvet Underground
The Stooges
In some ways, if you combined the three bands I mentioned above, I feel like you'd get the essence of what an American rock band is about: the power, the energy, the backbeat, the adventurousness, the attitude, the social commentary, the outsider's perspective, the danger, the volatility, etc.