Luther Lassiter won six major 9-Ball titles and ten major Straight Pool titles, all in the 1960's! He finished runner-up in either seven or eight (?) other similar events at Johnston City, the Stardust, The U.S. Open 14.1 or the World 14.1, going back to the 1950's.
In the modern era, Earl and Sigel won the most major 9-Ball tournaments. Earl won 50 major tournaments between 1982 and 2013, all were 9-Ball, except one each in One Pocket and Eight Ball. These were all full field events with most of the best players competing in them. Of course there were many more events for him to play in then during Lassiter's era. I wasn't able to find the records on Sigel, but suffice to say he won several dozen major events in the 70's and 80's including 9-Ball, 14.1 and even One Pocket. Nick Varner has also won over 30 major tournaments between 1980 and 2000. All three are far ahead of Efren in overall titles won. Efren was only dominant in One Pocket.
If you count major bar table events, then Buddy Hall stands alone. He won just about everything on the small tables for over twenty years (plus many big table events as well). These are the Big Four of the modern era, that also featured players like Efren, Parica, Mizerak, Davenport, Rempe and Hopkins. Johnny Archer became the dominant player in the 1990's (along with Earl), when we first saw the advent of the European players and more Filipinos playing on U.S. soil. Over the last ten years Shane has won far more than anyone else in major U.S. events, but his opposition in many of them was not nearly as deep or as strong overall (except for the U.S. Open wins).