It really depends on the definition of Rock. If you mean more classic rock then of course some of these would be different.
NOTE: these are my favorites not who I think deserve to be on the all time greatest list. Some of these bands make that cut some dont
In no order: The Band ABB Creedence Clearwater Revival American Aquarium Widespread Panic Skynyrd Led Zeppelin Doobie Brothers Pure Prairie League Joe Walsh/James Gang
It is pretty tough to put a list together.