In pool alone I'd have to go with Mosconi or Greenleaf, too close to call really but I think they're the only two really in the picture.
All cue games considered though, Walter Lindrum is the most dominant and by a large margin.
His autobiography by Andrew Rickett is a must read. It talks about his upbringing and how his father taught him to play and how diligent he was at such a young age. Basically Lindrum dominated to such a degree that he killed the sport. Joe Davis his main rival (if you can even call it a rivalry) popularized snooker and became champion at that because he knew he didn't have a prayer of ever winning a billiards title against Lindrum and he knew Lindrum would never have played snooker because he considered it far inferior a game compared to billiards. Davis was said something like "thank god Walter never took up snooker all we'd all be done".
Lindrum was so much better than the next best players that in exhibitions which took about 2 weeks to play, his opponents needed a 7,000 point spot and they usually still lost!
To put things into perspective, The highest any player besides Lindrum ever ran was 2,800.....Lindrum's high run was over 4,000 points.
Only a few players ever had a 2000 break (run)...(Davis never did) and the ones that did only did it once or maybe twice. Lindrum had over 40 runs of 2000 or more!
Lindrum was very firm in saying that it's what you put into the game that makes a champion, as opposed to natural talent. He always insisted that he was the best simply because he practiced way longer and harder than his competition. starting at a young age practiced at least 10 hours a day every day, sometimes 14 hours a day and he kept this up even after he was champion. If he missed a shot in competition he would practice it for 8 straight hours.
When asked if he thought anyone would ever reach his skill he said "no, because I really don't think anyone will put in the amount of practice that I have".
By the way, he played left-handed but was naturally righty.
The only other players I ever heard of that not just beat the best opponents but crushed them is Efren Reyes. Efren won one of the DCC 1 Pocket tournaments and the guy he beat in the finals was minus balls for 3 games.