It's hard for me to be to discern singers from music, or at least style of singing, I absolutely loathe Gnr but Axl has one of the widest range so I guess he would make the list, same for Mercury, I can't listen to Queen but he's one of the best singer, and probably an even better performer, in history, so if someone asked me "best singers" and I don't know if I would say those names from the top of my mind because I would think of what I like first.
I mean Stout has a great bass voice but so does Kennamer or Tim Storms, but honestly I can't feel a thing while listening to them, other than "wow they hit some impressively low notes". Sometimes they're so proud of their vocal ability that the music revolves too much around it and become pointless to listen.
My list is would probably be less rock-centric although rock (and derivates) is probably my most listened genre, without ranking them in vocal styles:
Kate Bush has one of the greatest and most impressive voices I've ever heard, I've heard a million of artists being compared to her, but no one came close.
Lisa Gerrard hit some unbelievable low notes and has one the most versatile voices ever, she uses it almost like an instrument, she has also one of the most expressive voices out there, it's not just academia (I don't think she was even trained).
Nina Simone is another one that I love,
Prince, although I'm not a super fan of him, is an amazing singer, so was James Brown
I absolutely love Scott Walker, from his pop days to his later experimental works.
Marvin Gaye had a spectacular voice, he could have sung everything. beautiful tone, always smooth, never pushing it.
Most people would list Jeff Buckley, who was an undebatably great singer, but I would probably go with his dad, years ago I didn't like him, found his style too "weird" but it grew on me.
Just noticed I'm just listing non- rock singers, I guess Mercury, Plant, Daltrey, Patton and Bowie would be my rock choices but David Bowie is probably the one I would listen the most, great bass voices, not the most impressive one in the bunch, but at the end of the day the music is more important