For me, it's Mike Sigel. He's the only player that belongs in the top five ever in both straight pool and nine ball, and he was top five during the heyday of both disciplines.
Then again, Filler or Gorst may cause me to change my mind soon.
By discipline, the best I've seen:
One pocket: Reyes
Straight Pool: Sigel
Nine Ball: Filler
Ten Ball: Kaci
Eight Ball: Reyes
Larry Nevel may have had the most powerful draw stroke I've ever seen, just a hair better than Strickland's. Mizerak might have had the best stroke ever, and if I may quote one of his contemporaries, he had the perfect blend of power and finesse.
Greenleaf and Mosconi predate my days as a pool fan.