Personally, I think there's a difference between singer and vocalist. Tom Petty is an incredible musician and songwriter and I love listening to him sing, but he's not much of a vocalist... and I don't think it matters. He sings his songs beautifully.
I know it's blasphemous to many, but years ago I became a convert to the rock bands of the 60s and 70s (I wasn't born when they were all big), but as hard as I tried, I just couldn't find it in me to like the Rolling Stones. And I think the main reason is because I absolutely can't stand to listen to Mick sing. I just can't get past it. Add in that he doesn't really play an instrument (harmonica doesn't count) and I just don't see any musical talent in him at all. He's one of the greatest entertainers of all time, so I can fully recognize his talents there. But I can't get past his voice and lack of musical ability. I could envision a scenario where a different singer and Keith Richards could have been just as big. I can't envision any scenario where Mick Jagger and someone other than Richards rising to that level. And it's not like the Beatles or Dylan, where the songs themselves drew me in and have kept them on top of most of my playlists. It's a point of contention with my in-laws who are like rock encyclopedias and know more about music than anyone I know, and are good musicians themselves. Nothing compares to the Stones for them. But they always tell me about how different they were they came out. "No one sounded like them" is what they say. I'm sure that's true.