Actual ranking shifts on kind of a daily basis according to my mood, so this is no particular order other than the order in which they come to mind...
The Rolling Stones
Booker T. & The MGs
Hell. I don't know if I can come up with a definitive twentieth. It kind of drops off after that point to artists who have some songs I like but of which I am not any kind of especially devoted fan. Aretha Franklin? Sam & Dave? Marvin Gaye? Green Day? The Foo Fighters? Pink Floyd? Led Zeppelin? I guess most any of them could fit in, but I don't know if I could choose just one...
Oh--wait. It just came to me. Knew I was leaving someone out. Smokey Robinson & The Miracles, definitely. There we go--that's twenty.