I think 'the greatest singer of postwar popular music' is a false construct ("in the history of" is just redundant), but if you had to choose one she'd be as good a candidate as any...BUT there is certainly no consensus on such matters that I've noticed, now more/less than ever. It's not as obnoxious and infuriating as the guy who said in the NYR that 'of course Cole Porter is a wittier songwriter than Chuck Berry', but it's not a particularly insightful or enlightening statement. Did he go on to do any close analysis of Aretha's actual practises as a singer, or does he just stay in the mist of offhand generality?