I haven't seen a lot of movies with dragons in them, but the one that still captures my attention and imagination is the one in Disney's Sleeping Beauty.
That dragon fills the screen with its aura of power and evil. It really is remarkable in that the drawing of it is simple without a lot of detail but enough to let you know it's a dragon. Not any any old dragon, but a big, big nasty dragon that you don't mess with. This is not a worm you have a friendly chat with or play riddles and forget about stealing any of its goodies.
This dragon will do a number of things to you: it will make you run faster than you think you can or make you immobile with fear or make pee in your pants.
My favorite dragon's voice is Richard Boone's Smaug. I believe Mr. Boone enjoyed doing the voice. Not thinking back on it I think it was the best part of the film.