I do not know why you would think that May is one of the greatest guitarists of all time, I do not find him as a huge influence and I never looked into his playing; he uses a coin for pick! Seriously Spud, maybe its just me though, for example a lot of people think SRV is the greatest but I always thought his songs were primitive and lacked variety.
I could think of some other bands/artists though, Elton John, Rod Steward, Creedence Clearwater Revival, Roy Orbison, Oscar Benton and maybe some others that I can't remember at the moment, Beatles maybe?