I can't think of anyone who even comes close to Bruce Lee.
Before him, there were only a very few people teaching Chinese Martial arts to non-Chinese in America... after him, almost all the schools opened up. His intro letters of westerners opened the doors of Hong Kong to several non-Chinese in a way that they had never before been.
I remember hearing about a guy teaching CMA in New Jersey before Lee. He was writing that when he heard Lee had been allowed to teach westerners, he got the courage to write to his teacher, cite the preceedent and begin doing the same.
I read a book on NHB (the old name for MMA) and read that the author, in the course of intervewing dozens of MMAists, hadn't found even one who, when asked, didn't think of Bruce Lee as an inspiration either in the past or in the present.
It was Bruce Lee who really lauched the idea of mixing modern western sports science (powerlifting, heavy bag, muscle recovery theories etc.) with Eastern martial arts... its hard to believe that so few did it before him. I have read about karate teachers only buying boxing heavy bags after seeing Bruce Lee doing kung fu on one... they didn't think it was compatable with TMA.
Much of the mutual respect between eastern and western martial arts actually comes from Bruce Lee and Gene Lebell. They were able to beat WMA using Western modified Eastern methods and then be sufficiently gracious in victory that it spawned a peace that still more or less endures. I have seen the scans of the sorts of stuff Eastern TMAists and WMAists were saying about each other from the fourties to the early seventies - when it really stopped being so prevalent.
What about the idea of using pylometrics in MA training? Bruce Lee was very possibly the first to ever do so in MA ... he did before the US Olympic track team had put it into practice after picking it up from the Soviets. His popularization of the method launched it in the MA world.
His mantra "seek the truth in combat" inspired generations of hardcore fighters who took what worked for them... and spawned the MMA revolution.
Then there is the matter of the entire art of JKD - which owes its existance to him.
Also, Lee is often considered (although I think he would have disagreed) the father of "RB"SD - the books published after his death that depict individual, very simple, gross motor techniques outside of any systematic organization are sometimes believed to have launched the RBSD movement.
Lastly, his high-profile publicity aided in publicising the martial arts.