This is a hard question because there are so many great martial arts movies. Can't name just one, but here are some suggestions in no particular order:
1. Anything with Bruce Lee - goes without saying really
2. Anything with Donnie Yen - Ip Man 1 & 2 (tells the story of Bruce Lee's real life teacher), Legend of the Fist, etc.
3. Tony Jaa - The Protector, Ong Bak Trilogy
4. Merantau - Indonesian martial arts movie that I saw recently. Incredible.
5. Five Deadly Venoms - classic
6. The 36th Chamber of Shaolin - classic