If you've watched the behind-the-scenes for Cradle 2 the Grave, when they were interviewing Jet Li for a brief moment of rest on the set (the cage scene w/ pit fighters)... Jet Li admits to more-than-likely getting his ass wooped by the pit fighters, should they actually go at it.
That can say something about his credibility as an actual fighter... but as an on-screen martial artists, it's pure beauty.
Jackie Chan's fighting is a lil' sloppier nowadays as compared to his younger years, but he has more than compensated by polishing his acting skills & increasing his stunt-difficulty.
Overall, I'm going to go with Jet Li. If Bruce Lee was assisted with the same bank & production budget that Jet Li has, he'd more than easily take it as the "best", he already has it as far as actual fighting skills & fitness is concerned.