Of course it is partly a matter of preference, but still, there is some objective elements as to which singers do have better vocal skills (at least that is how I interprete "greatest singers"). Dylan is a genius, probably one of the 5 most relevant folk-rock-pop musicians of our era, and a favourite of mine, but... seventh place in a "greatest singers" list seems like a joke. Conversely, Witney Houston had little relevance for the history of pop music but... oh, THAT voice!
However, I can agree with Aretha, Ray Charles, Sam Cooke or Otis Redding up there.
Oh, and sorry for my stubbornness. I used to be very keen on making lists...
I recently had a long discussion with some friends on "50 greatest Spanish sportsmen/women". Just for fun...
Can we agree on the 10 greatest jazz tenor sax players? Hawk goest first, of course...