The Allman Brothers Band
Credence Clearwater Revival
The Byrds
The Doors
The Beach Boys
The Ramones
ZZ Top
Crosby, Stills and Nash
Steely Dan
The Talking Heads
The Cars
Boston
The Eagles
Chicago
Tom Petty and the Heartbreakers
Alice in Chains
Nirvana
Soundgarden
The Smashing Pumpkins
Metallica
I could go on...
The Black Crowes
Grateful Dead
Jefferson Airplane
I'm bias, but I think the Grateful Dead is huge on here as they represent Americana. They played, Rock, Blues, Jazz, Bluegrass, Country, etc.
Nash was from England, so CSN was only 2/3 American.
Like @alumni94 , the Grateful Dead are my all time favorite American rock group. But I can't consider them the "greatest" because at the end of the day, they are still a cult band, albeit the most important cult band that ever was.