The movie doesn’t have to explicitly be about dragons, but dragons need to be featured as more than just a passing comment or minor cameo or scene.
I love the Eborsisk in Willow. It’s a very unique two-headed fire breathing dragon that is somehow both incredibly ugly and very cute, and Willow is one of my favorite films.
Rathalos from Monster Hunter and King Ghidorah from Godzilla are also very cool dragons.
I am ashamed to admit that I still haven’t seen these. I need to change that! I keep hearing that the first two are excellent, and the third movie and shorts are really good, too.