Greatest singer?
If you are talking voice alone, then Anne Murray was up there and Celine Dion...although not my choice, but if you want to get technical in what makes a great voice...Celine. There are others.
Personally on voice alone I would take KD Lang.
As an entertaining voice, I like Fred Turner from BTO.
Burton Cummings from The Guess Who.
Again in a technical sense MIchael Buble is very good.
Joni Mitchell is the most recognisable. She gets my favour because of her prowess as a brilliant songwriter and musician. Similarly to Bruce Cockburn, Chantal Kreviazuk, Gordon Lightfoot. And to a lesser degree, Nellie Furtado, the two guys from Barenaked Ladies. And like Mitchell, Paul Anka was a top-level songwriter, but he was like Buble, a crooner.
Alanis Morrisette is unique, but better as a songwriter than a singer. Like Jan Arden.
Now Neil Young is a great songwriter and a multi-instrumentalist, but on voice alone, he couldn't make it as a busker. Same could be said for a few other big names like Randy Backman, Gord Downie, Tom Cochrane, Colin James, Daniel Lanois, Margo Timmins, Emily Haines, Matthew Good, Jim Cuddy, Lights, Sam Roberts,
Diana Krall has her own voice. Great jazz pianist and strong voice. But if you want to talk jazz, then Holly Cole.
What about Rufus Wainright, Sarah McLachlan, Loreena McKennitt, Sarah Harmer, Jullie Black and Amanda Marshal.
Brian Adams' voice isn't good, but it is distinguished and popular....