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.
Reign of Fire scratches a particular itch that’s just neat for “dragon apocalypse in the modern day”.
Dragonslayer is classic 80s fantasy cheese complete with a grand battle on a mountaintop between an ancient wizard and the dragon itself, VERMITHRAX PERJORATIVE.
How To Train Your Dragon 1 and 2 are absolutely gorgeous with beautiful soundtracks, fun characters, and a story that goes straight for the heart.