That was Shishio Makoto, or Hitokiri Shishio. After Kenshin gave up the role of the assassin (he became more exposed), assassinations were delegated to a new, more talented swordsman named Shishio. It's ironic because I think Shishio's first mission was to execute the man who betrayed Kenshin and cost him the life of his wife, Tomoe. After that, he took care of a lot of...dirty business for guys in power and they were afraid that he would talk and Shishio apparently did threaten to reveal some of these secrets unless he was paid a large sum of money (according to the Meiji Government anyways). So they decided to silence him by bashing his skull in and burning him alive. However he survived the ordeal, his skin and flesh burnt, his sweat glands destroyed hence he could not release his internal body normally. Because of this he could only fight for 15 minutes ( a limit that he could have used more effectively if he didn't spend half of it monologue-ing) however he was still strong enough to take down all four of Japan's greatest fighters and did block Kenshin's strongest move. Shishio was not defeated, it was more like he was consumed by his disease in a crucial moment. In truth if it wasn't for his condition, he probably would have mopped the floor with Kenshin, Sano, Saitou and Aoshi. A badass to the end, he died laughing as flames devoured him. He later appears in Hell with the intention of conquering it.
I've always believed that Shishio is the best fighter in RK, however some of my friends think that Enishi is better. Total crap in my opinion but what do you guys think?