Depends on criteria. On tournament wins...as well as talent, dominance and such...
Earl Strickland is the GOAT.
But there will be bias against him due to people's opinion of him, as opposed to his game and his record. That won't change until a few decades down the road when his record will remain, not the drama.
A very close second is Efren. Efren has more diversity and success across more games, but does not have the number of big time titles that Earl does, nor was Efren ever as dominant in getting the titles he did. Earl was just a ball pocketing, perfect position play machine. What a freak. When he was in his prime, he steamrolled opponents.
After that, I'd say Sigel and Van Boening with Shane ahead. Shane can catch or pass Earl, he just needs a world championship at least, preferably two and another US Open. Earl has 3x world, 5x US Open.
Too much value is placed on Efren playing a variety of games. Is Mosconi's greatness diminished because someone else back then was stronger in 9-ball, one-pocket or 3-cushion? Nonsense.
The measure that matters is who was king and who dominated the main-game of the era.
9-ball was the game, and Earl owned it.
It's easy to understand why....Efren is likeable, a good sport, and not a man of much words and when he does speak, it isn't in a language most understand anyway. On the table, he is so absurdly creative, such immense knowledge. He is very, very fun to watch.
However, all those dazzling shots....while entertaining, most of them were recovery shots or because he was in a bad place that he shouldn't have been.
This is why in other sports, or even the arts ...there's fan opinions and expert opinion.
People get wowed by Efren's shots, but many of these come as result of a breakdown in consistency. He's human, everyone makes mistakes. But the greatest play is the most technically sound, and consistent play - which looks very routine and boring in a sense. Efren in his career rarely showed super consistent play, that also led to dominant tournaments. He wasn't the kind of machine Earl was.
Earl on the other hand has mild mental illness, and was heckled, trolled and triggered by vicious fans, leading to explosions and Earl being a sort of villain.