Of all time? Well...
Although Blue Cheer and Cream put on a heck of a show, too...
Blue Cheer was just Cream with sub-par musicianship (Tho they were loud)
I agree it's to hard to nail down.
Cream was always my favorite (don't forget how they got their name
) They weren't really ''rock'' in the truest sense, a bit of rock/blues/jazz/progressive but their long live jams with that "rollin' & tumbling'' sound as one critic put it has NEVER been duplicated. They were all sort of soloing at the same time but actually no one was soloing!
Arguably the Beatles...Stones, Zepplin, Pink Floyd, The Who...as far as rock and progressive rock go the English just seem to whomp us Yank's tails!
My personal favs over the years:
Beatles
Early Dylan
Doors
Cream
Rolling Stones
Chicago/Blood, Sweat, & Tears (their 2nd album could be my all-time favorite, not one note out of place IMO)
Neil Young
Emerson, Lake, & Palmer
Jethro Tull (Ian Anderson, what a musician AND SHOWMAN!)
Jeff Beck
Carlos Santana
Steely Dan (if you don't like them, you don't like music)
Van Morrison (for many, many years) etc...
Jazz has always been my favorite tho, that's why I always liked the "rockers" that expanded into jazz or fusion in the early-mid 70's, Santana, Beck, Steely Dan, Soft Machine, Gary Moore, etc...
Now I'm finding myself more and more listening to a growing classical collection (Baroque (Bach/Handel/etc..) & Impressionism (Debussy, etc...) being my favorite genres along with Gregorian Chants, Enya, Japanese Flute/Harp....soothing stuff.