Brazilian Jiu Jitsu, basically the reason the UFC was made and the most common ground fighting system used in MMA.
Carlos and Helio Gracie adopted an altered version of the traditional martial art and developed it, with almost their entire family learning it and it being passed on to every newborn. Royce Gracie entered the first UFC when it was style vs style and just battered everyone despite being probably the smallest man in the competition. It was set up at least in part by Rorion Gracie to display how effective BJJ is.
Google the Gracie family, they're surely the hardest family on the planet. Renner, Rickson, Renzo, Kron, Kyra, Royler, Royce, The Machados etc. are all serious, serious martial artists.