In a kickboxing match in a prize ring probably Bill Wallace.
In a fight without any rules on the street probably Bruce Lee.
@ Shinobi: The "fight" between Lee and Wallace you´re talking about actually was a sparring match. Lee never pushed the limit regarding groin kicks, hits to the throat or fingerstitches to the eyes because he knew it would be too dangerous. He sparred a lot of well known martial arts masters to learn and improve and many people claim he lost but I wouldn´t even consider these matches real fights at all. By the way Lee would definitely do much better in a barehanded street fight than in a ring.
edit @ shinobi again: Don´t you see the difference between a friendly sparring match just to test each others skills and a real fight? Bruce Lee trained to attack groin, throat, eyes, neck (which he without a doubt didn´t do when he trained with Wallace) and considered all possible methods in a self defense situation to survive. He would have killed Wallace if it would have been to the death - but of course it wasn´t as both respected each other and there would be no reason to destroy. By the way have you any proof that Wallace beat Lee in an actual fight? No.
edit @shinobi: You don´t know me as a fighter and not as a person also. 6 years of training? That´s actually the same time I train boxing and martial arts. I don´t consider Bruce Lee an invincible fighter as nobody is. He´s human and had flaws in his game, like everyone else. I of course know JKD is not a style, why do you think I thought so? And regarding the groin and eyes as JKDs main targets: It is true. Bruce Lee himself said so in one of his books!! He considered groin and eye attacks most effective and important in real fights (to the death/to defend your life). Bill Wallace is a great fighter and I don´t doubt his overall skills but he never had Bruces speed, explosiveness or power. Lee P4P could have been one of the most powerful men in the world. I don´t claim so, but the humans who were closest to him do! Regarding defensive against usual combat sports kicks and groin kicks: I trained with self defense experts and I myself also made the experience that the movement to block it is very different as it comes from a different angle which makes it very hard to avoid it, especially if you´re used to fight under rules. You say it won´t make the difference? Ok, then let´s look at mixed martial arts. How many situations in grappling on the ground or in the clinch can you remember where an opponent could have pushed a finger into his opponents eyes or nail his knee into his groin, or bite, or attack throat, or hit or grap his nuts to rip it out. Yes, hundreds - in any fight. But there are rules. It is disallowed. It changes everything. Also in the stand up. Groin kicks even happen unintentionally. And they wear nut protection and it still hurt so much that in most cases they need a break before continue to fight. Now also Lee had amazing deception maneuvers, that´s why he would find the target. Of course Wallace had a much better kickboxing technique and I think he´d win in the ring, as I said. On the ground Lee would again find eyes, throat or groin (he can´t just attack it in the stand up with punches and kicks, but on the ground also with graps, in almost any situation) and he would bite to come free. Lee could push his finger through a canister, he would have been able to push a finger through your eyes into your brain, no joke. He was in shape 365 days a year, nobody came close to his passion. He was fanatic as nobody else. I didn´t always say that I consider him the most dangerous fighter I can think of, I studied this man overy years to try to find out how good he really was. As so many people said he was the best and on the other side so many said he was an overrated MA and just good as an actor. Now this all is still speculation, we will never really know. And by the way it is unimportant for us. So please believe what you believe and let others think how they think. Wallace and Lee respected each other, so please let us respect each other as well. I totally accept your point of view and I won´t get personal just because of this stuff. Best wishes from my side.
Please read Dalmatiners edit in his answer to this question also.
It´s easy for the Bruce Lee haters to come here and claim he was overrated but fact is all those people who knew him best - family, friends and training partners - all agreed he was one hell of a skilled martial artist, including Bill Wallace who was very impressed by Lee. Wallace said: "i don´t know how good Lee is as a fighter, as I never saw him in a fight. But his technique was superior"
By the way excuse my english, I´m german