Is he the greatest rock vocalist of all time?
Of course. No one had or has the same vocal range than him. You can talk about sentiment and those things that fedoras talk about, but if you say that Dylan, Cera or a random punk guy is a better singer, you're objectively idiot.
Is Rock 'N Roll a different enough genre to exclude it (and therefore Elvis) from this discussion? Otherwise I concede that it's at least a tie, if not in Elvis's favor. But Freddie was a better writer.