From what I've seen and heard, he was extremely good in many aspects of his game (even by today's standards), but had room to improve in certain parts (like the grappling).
Unbeatable? No one is unbeatable.
Another Jackie Chan or Van Damme? Bruce was a real fighter: pretty complete by 1970's standard, and quite good even by today's standards. I haven't heard anything about Van Damme's nor JC's fighting prowess.
To me, Bruce is unbeatable in terms of his contribution to martial thought and practices. He was one of, if not the first, to truly advocate physical athleticism in the martial arts (back in the day when "ki" or "chi" were the rage). He was also one of the first to advocate studying several martial arts in order to attain complete martial knowledge. To me, this is his lasting legacy.
I can think of several fighters who would give him a hard time, if not outright KO or submit him.
But I can only think of a handful of martial artists (Kano, Gracie, etc.), who really shaped and revolutionized how we train martial arts today. Bruce Lee is one of those martial artists.