Just about every single mythology has its own trickster. Tricksters aren't just pranksters. They possess an intense intellect or a secret knowledge that could bring everything down. They also don't obey rules or conventions. Here are some of the best.
There are a lot of dragons in Norse mythology. Perhaps the greatest is Jormungandr, the World Serpent. The second greatest is Fafnir, who fought the great hero, Sigmund.
But the third greatest (and relishing in his lowly position) would be Nidhogg (also called Níðhöggr).