There are many greats mentioned in this thread, and many more yet to
be mentioned. But I'll take a different tack.
When I think of a singer whose voice has *character*, I think of
someone like Johnny Cash.
When I think of singers whose voices created a unique and immediately
recognizable blend of sound, I think first of The Four Tops, The Mamas
and the Papas, and, to a lesser extent, X.
There are many different ways you can look at this question.
To the OP, give a listen to Maddy Prior (from Steeleye Span).