Who's the best martial artist of all time?

Kanbun Uechi was pretty bad ***. Legend has it he killed a man in China in a dispute and had to flee back to Okinawa.

George Mattson is also a living legend and a 10th degree black belt in Uechi Ryu.

Shinyu Gushi was considered a "living legend" when he was alive, and award 10th degree black belt after he passed away.

In other words, many of the best in the world most people have never heard of. And these martial artist are the "best" in ways that people like OP don't understand. We can't rate if Gushi was better than Bruce Lee. Or if Mattson was better than Chuck Norris. I can however tell you that they've all contributed in many ways just like I have, and many of the very senior folks who contribute here.

Most likely someone who isn't famous and probably died a long time ago or has yet to be born

Bruce Lee for kung fu, but it's Jon Jones for MMA