Bruce Lee
Royce Gracie
Ken Shamrock
Frank Shamrock
Kazushi Sakuraba
The first three are obvious, Art Davie and many others in the MMA community have named them as single entities who changed martial arts (though with Royce it was a family achievement) through cross training, ground, submission etc. Frank was a prototype mma fighter from the first era into the next 1997-2000, and was the first 'complete' fighter. Saku is Saku.
Arguably, anyone afterwards... well how can someone be as influential on mma popularity and fighting evolution as these men? As good as any predecessor like Fedor may be, this is it for me