Bruce Lee at the time was the most skilled fighter on the planet because he started doing what MMA fighters are doing now, he just did it first. With his skills at the time of his death he would get smashed in the UFC. Had MMA been going on at the time and he had the opprtunity to train in the multiple facets of the game he would've been a 135lb Anderson Silva. Bruce Lee was never a stagnant martial artist. His drive and determination to constantly become better and learn new things is shown best today in guys like GSP and Anderson. There's no way in hell that, had Bruce lived, he wouldn't have just kept getting better and better and I think it's absolutely preposterous to label it as such. I really wish people would stop looking at Bruce as just the fighter he was when he died, not the fighter he would've been had he lived. This was a guy who INVENTED a shit load of workout equipment, who worked out while filing business papers and making phone calls and who went around to other dojos and gyms trying to learn techniques. There was no one more dedicated to being a martial artist than he was and there quite possibly never will be.
I'm aware of why he's special. I've read the Tao, as well as seen just about every documentary I could find.
That's why I give him the early UFCs. But he could most assuredly lose, never forget the cliche "its MMA anything can happen" remember. Who's to say he wouldn't possibly get outgrappled by Gracie (the family would likely have sent a better Gracie had they known the greatest Martial Artist of all time, at the time, was competing).
If he was a young'un when the UFC first became popular and he started becoming a total fighter as a youth, I have no doubt that Uriah Faber wouldn't be slotted to fight Dominic Cruz. But for the sake of logic lets assume he has the skills he had in 1973. Against Dominic Cruz, Uriah Faber, Jose Aldo, and Mark Hominick (referencing fighters of comparative notoriety at similar weight) he'd be lucky to survive the first round.