Depends if you're talking about serious films, chopsocky kung fu films, or American made "inspired by Hong Kong" martial arts movies.
Yojimbo and Sanjuro by Akira Kurosawa are a good start. Also Seven Samurai, really it's hard to go wrong with Kurosawa. Onibaba by Kaneto Shindo although it's less of a martial arts movie and more a movie that takes place in the shadow of the Sengoku period.
There's a lot to choose from. There's One Armed Boxer and it's sequel Master of the Flying Guillotine. Invincible Armor is pretty wild. Tiger & Crane Fist that you probably won't enjoy as much as Kung Pow!
Any of the Godfrey Ho movies. The Little Hero of Shaolin Temple which is sometimes mislabeled as Little Ninja Heroes and has nothing to do with ninja if I recall.
There's the American Ninja films. The Last Dragon. Big Trouble in Little China, which is pretty much responsible for Mortal Kombat. Speaking of which if by some chance if you still haven't seen it the original Mortal Kombat is a really good American made martial arts movie. Bloodsport. Don't watch any of the Steven Segal movies.