This poll offends me (jk)
Seriously, Romeo Must Die? Of all the awesome Jet Li movies to choose from, why would you put up one of the worst ones? I think that this is no one's favorite movie, anywhere. Other folks have already mentioned some of the greats...Fist of Legend, Once Upon A Time in China, Fearless. Tai Chi Master is another. Even Kiss of the Dragon would be better that RMD. I love Unleashed, but I don't really see it as a martial arts movie so much. Same with Hero.
You missed several great Shaw Brothers classics that could easily be someone's fave's. Shaolin Challenges Ninja aka Heroes of the East, Master Killer aka 36th Chamber of Shaolin, as someone already mentioned.
Whoever said Enter the Dragon is the greatest empty hand martial arts movie of all time must either be in love with Bruce Lee, or hasn't watched any movies since 1975.
Sure, it's a classic, but watch it along side any of Yuen Wo Ping's work, from the seventies on through the 21st century, and it just doesn't hold up.
A list of great martial arts movies can almost read like a filmography for Yuen Wo Ping. He's either been the director, choreographer, or producer/consultant in some capacity on almost every good martial arts movie in the last 30 years. That's a little bit of an exaggeration, but he does have an impressive body of work. If you don't believe me, look him up on the internet. If Master Yuen's name is involved with the movie, it's probably worth at least one watch.
That reminds me that Iron Monkey isn't on this list, a great one with Donnie Yen.
Liu Chia Liang is another great choreographer and director, from back in the Shaw Bros. days. He's responsible for many of the best ones.
You also missed some great movies from Jackie Chan's opera brothers, Sammo Hung and Yuen Biao, like Prodigal Son and Magnificent Butcher.
I can't decide on my all time favorite, there are too many different types of martial arts movies to lump them all into one category. Do you grade on pure martial arts, just the choreography and style of the fighters? Are you grading on actual story content and film quality? What qualifies as a "martial arts" movie? Any movie that has fighting in it? That would be just about every action movie ever, including a lot of westerns.
If you've ever practiced two person fighting sets, empty handed or with weapons, then you have an idea how awesome some of these choreographers and actors are. Even if the story and acting in a movie is completely cheesy, I watch them just for the choreogrpahy.
watch Liu Chia Liang and Hsiao Ho in this scene from Mad Monkey Kung Fu. I bought the DVD pretty much for this alone.