No real criteria... don't have to be the 5 best ever... just ones who are especially good, whose voice is special (in a good way)... who you can tell who it is right away...
(doesn't have to be just rock either)
In no particular order
* Lennon
* McCartney
* John Fogerty
* Jack Bruce
* Roy Orbison
* Joe Cocker
* Janis
* Rod Stewart (even though he wasted his talent after the first few years)
* Levon Helm
* Steve Winwood
* Johnny Cash
* Willie Nelson
* Tracy Chapman
* Dylan (because it's very important to some of his songs)