I've never really liked Elvis, actually. I find his vocal style, well, annoying. That's not to say he was a *bad* singer, but I think there are/were plenty of better singers out there. I'd much rather hear Sam Cooke, Otis Redding, or Ray Charles than Elvis.
I agree that all three are better singers than Elvis, but I'd call those fine gentlemen soul/R&B singers, not rock singers. (Though you could make an decent argument using "Otis At Monterey.") In fact, I'd commit the heresy of suggesting that Elvis himself was often more of a pop singer than a rock singer. (But he could certainly sing rock, as in the early singles and the "comeback" special.)
But Elvis ranks high with me anyway. His command, confidence and sheer vocal charisma are devastating. The problem is that the post-Army years had a lack of good material.
As far as "rock" singers:
Technically, Freddie Mercury was the man. Anyone who could handle such disparate material as "You're My Best Friend," "Tie Your Mother Down" and "Who Wants To Live Forever" with equal aplomb merits at least a mention.
My non-expert opinion is that Jagger and Daltrey impressed more with attitude than skill, no Daltrey in his prime was no slouch. (I've never heard truly impressive
singing from Jagger, especially live, though he's a rock frontman without peer.)
Dylan is the king of phrasing. The meters and rhythm he uses are astonishing. Something like "Subterranean Homesick Blues" is rap-like in its complexity.
Let's see, who else in "rock"? Plant was a great wailer, and probably the most-imitated singer in rock, so that's gotta count for something. Jim Morrison and Eddie Vedder have a similar sort of "power brooding" style that's effective with the right material (and Vedder's probably the most-imitated singer in rock right now).
In the end, though, I think we all have to bow down before the awesome majesty of Lou Reed.
Ryan