True, Rudy... within TKD. In terms of most well-known overall, though... probably Jhoon Rhee. How many times have you changed the channel to see him doing pushups? Or showing off his silk dobok?
Quite right Jion:
Master Rhee is an icon, and an amazing individual. Not as well publicized here in Canada (so we don't see him on TV that often), but nevertheless very well known. I had the great fortune to meet him when I received some hardware from him at the North-America TKD championships back in the early 70's. He is quite a humble person too.
Rhee gets press here all the time, or used to, for teaching various government officials. I believe his main school is in the Washington, D.C. area, so we're talking congressmen and senators. Most of his television appearances are more a promotion of physical fitness than martial arts usually. He may be a part of the Presidential Physical Fitness committee, like Arnold was.
Famous and influential are not the same thing. As far as strictly famous, Hee Il Cho ranks pretty high up there, since he videotapes and sells pretty much anything he ever does.
As far as influential, I think it's a tie between Hong Hi Choi and Jhoon Rhee. Both brough Tae Kwon Do to the United States and the world.