I will steer clear of commenting on Bruce's personality and focus on his fighting ability and legacy.

Bruce Lee is a very complicated figure in the martial arts community. As one of the highest profile martial artists of all time, he did more than anyone to spread the good word of eastern martial arts in the west. His movies, his philosophy, his brand aided in the explosion of eastern martial arts in the United States and abroad.

In many ways, Bruce Lee was a man ahead of his time. His denunciation of dogma, his appetite for techniques from different martial arts, his approach to training, his integration of strength, conditioning, and nutrition into his overall philosophy, all of these things are reflected in the Modern Martial Arts landscape.

Unfortunately, his status as a pioneer in the mixing of martial arts is what makes him so difficult to evaluate as a fighter. The same year Bruce Lee arrived in America in 1959, a brazilian family called the Gracies began producing a Rio-based TV show called "Heróis do Ringue". While mixed rules, catch as catch can fights had long been carnival mainstays, it would be decades before the establishment of promotional pioneers such as Shooto, Rings, and Pancrase. The first regulated Mixed Martial Arts bout in California wouldn't occur until 2006, many years after Lee's death.

Similarly, American Karate competition was in its infancy when Lee arrived. Following the end of the second world war, Karate began to appear in the continental US, brought back from the Pacific and Hawaii by servicemen such as Robert A. Trias. Trias is credited for organizing the first Karate competition in the US, the 1955 Arizona Karate Championships. The first national competition in 1963 at University of Chicago's Fieldhouse and the subsequent U.S. National Karate Championships in Washington, D.C were disorganized and limited in scope. American kickboxing also had yet to find its feet. The PKA and WKA would be formed in the mid 70's, coming too late for Bruce, who passed in 1973.

There are many anecdotes on Bruce's personality, his fighting ability, and his philosophy. Perhaps his most legendary meeting was with Chinese-American martial artist Wong Jack Man in 1964. It is very much a story of two sides, with neither side presenting much in the way of evidence. Bruce's wife Linda said

The fight ensued, it was a no-holds-barred fight, it took three minutes. Bruce got this guy down to the ground and said 'Do you give up?' and the man said he gave up

Others, including Wong Jack Man, allege that the fight ended after 20-25 furious minutes due to Lee's "unusually winded" condition, as opposed to a decisive blow by either fighter. What is not up for dispute is that Bruce Lee changed his fighting philosophy following that day, formed Jeet Kune Do, and in the process changed the story of American Martial Arts.

Bruce Lee was not a professional fighter. Unlike his student Joe Lewis, we do not have an extensive list of bouts won and lost. He was never a champion, he never defended a belt, and he never retired from professional sport. He was an actor, he was a teacher, and he was a life-long martial artist. Without competition record, there is very little we can definitively say about how good a fighter Bruce Lee was. At 5'8'', 141lbs, Bruce was no heavyweight. Common sense indicates that two equally skilled fighers, all other aspects being equal, the larger man will win most bouts.

Conclusion

I think it reasonable to conclude that Bruce Lee could defeat well-trained period martial artists near his weight, and larger, untrained opponents. However, it is equally reasonable to conclude that he would struggle to defeat larger, well trained period opponents such as his student Joe Lewis (heavyweight kickboxer with training in wrestling, karate, and boxing).

As an aside, I believe Bruce would struggle to compete with modern top MMA of any size. The number of participants, the level of training available, and the evolution of technique over time has changed the sport into a form that would be unrecognizable to earlier audiences. If a young Bruce Lee walked into a gym in San Jose or Huntington Beach tomorrow, now that is a completely different story.