I don't know if you could say someone is "the greatest musician ever." Maybe within a genre of music, but not over the whole in a general sense. Mozart might very well have sucked at writing Rock & Roll had it been around in his time.
As for the music I listen to, I would name these following as great:
Pink Floyd, particularly Roger Waters
Bob Dylan
Neil Young
Led Zeppelin
Rachmoninoff
Beethoven
Chopin