Smaug (The Hobbit)
I don't know if you would consider this dragon to be particularly fierce so much as misunderstood, but Smaug did apparently destroy a village and take control of the mountain where all that lovely treasure was buried. I consider that to be rather fierce.
Unnamed Dragon from Beowulf
While it's true that there is only one story of this dragon in existence, it's still a pretty good story that reminds us how dangerous and powerful this creature really was. Not only did it destroy everything in sight, but it even managed to kill Beowulf, albeit after its own death. Considering that Beowulf was pretty much the best thing since sliced bread back then, that's impressive.
The World Serpent, or Jormungandr from Norse Mythology
Why on this list? Because according to Norse myth, this dragon is supposed to engage Thor in a final battle during Ragnarok (or Norse apocalypse, if you will), and in doing so he will poison the skies and the seas. While the World Serpent is supposed to lose to Thor, his poison will eventually destroy Thor and, well, you can imagine what other horrible things will occur after. It's hard to survive in a world that has been poisoned, right?
The Dragons from Reign of Fire
While the movie itself wasn't all that good, the dragons were formidable enemies. These creatures breed like rabbits, lay waste to pretty much everything they come in contact with, and seem to have only two modes: kill and spit fire (which sort of go hand in hand, I suppose). Nasty beasts, for sure.
Vermithrax Pejorative (Dragonslayer)
I don't know how many of you have actually seen this movie, or have even heard of it, but Vermithrax was a remarkably ruthless creature with a lust for virgins. His name actually means "the Wurm of Thrace who makes things worse." Much like the dragons from Reign of Fire, Vermithrax is pretty much interested in only two things: killing (virgins) and spitting fire. There's some reproduction and general desire to be mean thrown in there for good measure, too.
Maleficent (Sleeping Beauty)
While technically she's a witch, the fact that she turns into the dragon at the end lets her be on this list. When I was a kid, that final fight scene actually scared the hell out of me. I don't know why, because you'd think even kids would be able to tell the difference between cartoons and reality, but Maleficent was a terrifying, monstrous creature with astounding power. That's good enough to put her on this list.
Godzilla (from all the Godzilla movies)
Terrible temper + ocean-side city = smash and crunch. How many Godzilla movies are there anyway? Something like thirty-eight, actually, each one involving this skyscraper-tall monster smashing cities or beating on other giant monsters. I can't think of any moment in which Godzilla was even remotely liked by the poor people of Japan. Imagine having this guy as your next door neighbor!
The Hungarian Horntail (Harry Potter)
I'm not going to say anything at all. I'm just going to point you to the video below. That should show everything needed to be said as evidence of this creature's fierce personality.
Honorable mentions: The Eborsisk from Willow, Glaurung from Tolkien's Middle Earth, the dragon that Saint George kills from European mythology, Fafnir from Norse Mythology, and many others. It's quite difficult to make a list like this! There are so many dragons both mythological and straight-fictional to choose from.
Which dragons would you put on this list instead of the ones I have there and why? Feel free to disagree with me!