Well it's hard to really choose a fighter from the right discipline, as I am not sure what type of fight you would put Bruce Lee in.
What I can say is.
Put him in a boxing match with Floyd Mayweather, and he'll lose badly.
Put him in an MMA match with Anderson Silva, and he'll lose badly (not a fair fight though, as Silva is like 6 foot, but you get the idea).
Put him in a kick boxing/Muay Thai style fight, and there's bound to be people who would beat up on him.
He was more of a showman than a fighter.
Of course if you take the fighter out of their element they're going to get their ass kicked. Bruce's style didn't adhere to any rules. Rules are important for any competition, naturally, but once rules are established certain techniques are banned and thus other techniques that have primary weaknesses that are banned become more powerful as a consequence. Don't believe me? Alright I'll explain.
Example: Double leg take down
A great technique for certain. But it's used all the time in mixed martial arts, sometimes in situations where it wouldn't be wise in an honest fight. In the octogon you can't strike the back of the head, when attempting a double leg take down this is the most exposed part of the head. Once one of their knees is on the mat you can't kick or knee them in the head. You can't grab and rip off their ears, it could be easily done in defense of a double leg take down, because the ears are exposed and their arms are away from their head. You can't break their neck. You can't gouge their eyes with your thumb. It's against the rules to do these things, but they're all effective ways to defend the technique. So the technique becomes dominant, because it's superior in the meta-game, the established perimeters for the fight. But in an honest fight without rules, without limitation, not so much.
Don't misunderstand though, I'm not saying it's a bad technique, because it's not at all, it's an amazing technique. I use it all the time while sparring. But I'm constantly protecting my face and the back of my head, I'm constantly trying to apply it in a way that ensures my safety while still accomplishing what the technique is designed to do. Because when I'm sparring with my partner I trust them to not intentionally harm me, so we have no rules at all. It's much harder to apply this technique when considering all these other obstacles and hard ground is a lot more intimidating than the flat mat of the octogon.
This "Meta-game" effect, can teach fighters bad habits. Why, if I'm concerned for my safety, would I practice a guard that assumes I won't get hit in the testicles?
It's funny when you consider that MMA directly defies the original spirit of mixing martial arts in this way, something Bruce was all about. His self invented martial art "Jeet kun do" was a mesh of many things he'd been trained in "From boxing to jujitsu". But wasn't so much a style as a philosophy of how to develop your own style. He taught "Take what is useful for you and discard what isn't". Basically it was MMA before MMA was even a thing.
Saying he was more of a showman is ironic.
Anderson Silva would probably destroy Bruce Lee, but Floyd Mayweather might have to work extra hard to take out Bruce Lee. Bruce Lee utilized his knees and elbows along with his fists. Although Floyd has great defense as a boxer, he might not be disciplined enough to move around other things that aren't fists.
Well, we'll never know the honest extent of Bruce's abilities since he's dead. But I'd wager he could've stacked up to just about anyone in his weight class.
It's easy to say that these fighters are better, because we've got a much larger archive of their abilities to access. Comparatively we know little of Bruce's abilities, we know little things, like he was too fast for cameras. Basically the cameras couldn't capture his movement hardly, he had to slow down to be a blur.
But at this stage that's all exposition.
I'd love to have seen Floyd fight Bruce in a real fight, where he doesn't have bitch mittens strapped to his hands and can't inject his fists with painkillers.
