That's a tough question. I assume we are talking only "bands/groups" as opposed to duos & single artists.
I agree with many posted. In particular for me, The Doors, The Eagles (saw them twice. Great concerts), and of course The Beach Boys, Fleetwood Mac. Also, tho being from the state (MI) I still think Motown bought us some of the greatest like: Four Tops, Temps, Supremes, along with Bob Segar & Silverbullet Band, Mitch Ryder & the Detroit Wheels.
Honorable mentions also to Santana (is Carlos Santana American?), Iron Butterfly, CCR. Oh, and I always liked the "Guess Who" but I think they are from Canada.
I know I am forgetting many other great groups but it's a start.

TIS