Steffi Graf is probably #1.
Most dominate in Cricket and Jai Alai
Sir Donald Bradman
Jose Antonio Illoro (Bolivar)
If we are talking about the most dominant athletes of all time, then many are talking about Phelps, Jordan, Gretzky or Chamberlain, but there were more dominant athletes out there. Bradman was really dominant in Cricket for sure. I have to read up on the Jai Alai guy, but being dominant in such a fast sport is impressive.
Speaking of dominance.
Aleksandr Karelin (Russian Wrestler) - He had a wrestling record of 887 wins and two losses. Both losses were by a point. Opponents failed to take a point from him in six years. I think he was undefeated for a 13 year stretch at one time. That is quite a remarkable feat. He was a beast !
Masahiko Kimura (Japanese Judoka) - Awesome judoka who only lost four fights in his entire career. All losses came in 1935, and he avenged all of them later in his career. Most famous for beating the "godfather of BJJ" Hélio Gracie in Brazil. Pound-for-pound he had no peers. Today, the "Kimura" reverse arm lock is named after him.
Reinhold Messner (Italian Mountaineer) - His will power and physical endurance is legendary. He was the first climber to ascend all fourteen 8,000 metres (26,000 ft) peaks. On top of that he did them all without supplemental oxygen, which is quite remarkable. An absolute tour de force in the days when the equipment wasn't what it is today. Made many first ascents. And he survived !
Jahangir Khan (Pakistani squash player) - Went unbeaten for 555 straight games between 1981 and 1986. Khan had superb conditioning and was quick as a weasel. He absolutely dominated the sport, and to beat him twice in a row was almost impossible. Khan was already the amateur World Champion at the age of 15.
Edwin Moses (American track & field athlete) - The guy won 122 consecutive races, including 107 finals. He did not lose a 400 m hurdles race in nine years, nine months and nine days. Technically flawless and had perfect timing.
Mike Tyson was a dominant force in the early years, and is a candidate when looking at peak value. But both Tyson and for example Rocky Marciano both only fought a few really good fighters. Usain Bolt is also someone who was quite dominant for years, beating the best of the rest all the time.
I am sure that I forgot about someone, but the five above are the ones that came to my mind when speaking of dominance in a sport.