The greatest rock and roll singers ever are as follows:
Freddie Mercury
Geoff Tate
Robin Zander
Grace Slick
This list is in no particular order, but let it be known that Freddie Mercury is always number one. Always. The other three can share the number two spot, it's not that important. But they will never bump Freddie from his rightful place at the top.
By the way, I am not talking about who is the greatest entertainer, or the greatest frontman, or the greatest showman. I am talking about singing. My criteria is simple. If you removed the music from a song, would this singer be hitting notes on key? Then they are a great singer. Bonus points are included if the singer has great power or great range in addition to the ability to hit notes.
Ringo Starr is not that strong a singer. He doesn't fit any of the criteria. I don't think I'm going to find anyone who wants to argue that point. Same goes for Vince Neil, Bruce Springsteen, and Paul Westerberg. These three are great entertainers. Bruce and Paul are great songwriters as well. But they do not fit the bill for great singers.
I saw Grace Slick, a member of my list, once on television. She claimed that yes, she is a good singer, but has limited range. She then illustrated her point by singing in a high register. It wasn't pleasant. The power was not there, nor was her note-hitting accuracy. However, I would have never known that about her if she had not demonstrated. Grace knows her range, sticks to it, and sticks to it well. So she has a spot on the list. Just try to listen to a Jefferson Airplane song without admiring her sound. You can't do it!
Geoff Tate is on the list, but only Geoff Tate from 1983-1990. For non-Queensrÿche fans, this means it goes from the self-titled EP up until the Empire album. I'm pretty sure he still had his voice for Promised Land, the 1994 album, but it was such a piece of crap that I don't have it anymore, so I cannot speak with any authority about that record. And in these modern times, Geoff's voice is starting to go. I have heard Live: Evolution, their most recent release, and it is not a good thing. The high notes do not belong to him any more. And since there has been a decrease in power and range, present day Geoff does not qualify.
Robin Zander is also a bit shy these days about hitting the high notes, but he knows it is not his strong suit anymore, and manages to avoid them. And it has been such a long time since Cheap Trick released a proper studio album, I don't know for sure how his voice is holding up. But he is still doing it up well at the live shows (minus the high notes.) Robin Zander qualifies for the list from 1977-1997.
And Freddie Mercury. Dear, dear Freddie Mercury. From 1973 to 1991, there was no one who could touch him. There is still no one who can touch him. An unparalleled singer. I know I said I am not talking about who is the greatest entertainer, frontman, or showman, but Freddie happens to be number one in all three of those categories as well. You could remove the music from the entire Queen catalog, have a listening marathon, and never hear a bad note. (You should go listen to the entire Queen catalog anyway.)
Before I go off on a long Queen tangent, let me sum up. Freddie, Geoff, Robin, and Grace are my top four. I have listened to lots of other singers, hoping to find more for the list, but none have presented themselves yet. You would be surprised at how few truly great rock singers are out there.
Postscript: I know that you are all compiling your own lists right now. You are saying to me, where is Paul McCartney? Where is Steve Winwood? Where is Jim Morrison? Okay, maybe no one I know is asking about Jim Morrison, but still. Paul has a nice voice, but nothing that elevates him into the category of "greatest singer ever." Steve Winwood. I love him. But he is not the greatest. Jim Morrison, good voice, nice tone. Always hitting notes? I don't think so. So stop and take a good listen to your favorites before you send me any huffy emails.