The problem with determining the best ever in pool is that there are so many games in which to judge the best. You almost have to go by category.
Efren comes out on top in one pocket, head over heels better than anyone, and from there it can be a toss-up for who's the best in a particular game.
You can't really place Willie Mosconi in an all round category, because he mainly played straight pool.
The same could be said for Buddy Hall, who is arguably the best nine ball player ever (even though he thinks Lassiter was the best in nine ball). Buddy played great one pocket and banks as well, but not a lot of straight pool.
My all round picks would have to start with Efren, and then Sigel or Lassiter, Harold Worst, Nick Varner (was playing some great pool in the late 80s, and even beat Efren in the Philipines in a long race and for the cash), and Steve Mizerak. Mizerak was the man to beat in nine ball for a while.
Jim Rempe would be somewhere in the top ten, especially the way he played nine ball in the 70s.
From what I've noticed in pool, a lot depends on who is hot at the time.
No one stands out like a Tiger Woods in golf, except possibly Efren in one pocket