1. Royce. He introduced most of the world to BJJ. He probably inspired more people to try bjj than any other athlete.
2. Dan Severn. First wrestler to really show what they could do in MMA.
3. Bas Rutten. This guy was the whole package. As many submissions as KO's, feared striker, competent on the ground, and decent wrestling.
If we were talking people that never fought in MMA, then guys like Judo Gene Labell, Bruce Lee, and Frank Gotch would have to be mentioned.