IMO Mosconi may well be the greatest straight pool player ever. No one has ever been more of a perfectionist or more of a ruthless competitor than Mosconi.
But, I also think that Greenleaf sometimes gets overlooked in discussions of the greatest players ever, partly because his heyday was the 1920's and 30's, and partly because his greatness is somewhat overshadowed by his eccentricities and his battle with the bottle. Mosconi and Greenleaf are, quite possibly, the two greatest straight pool players ever.
I've had two people who know pool and said they knew Willie and saw him play NINE BALL and said he was also the best nine ball player that ever played the game. What do I know? I was just a baby when they played.

JoeyA