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.
The GoT dragons were pretty great too, but the series is getting a bit old at this point. They looked awesome for the time but HotD steps it up visually to a whole other level.