Smaug - While not the most powerful dragon in Middle Earth, Smaug is the most famous.  That's because he rose toward the end of the Third Age and crossed paths with Bilbo Baggins, the hobbit who rediscovered the Ring of Power.  It was Gandalf who assembled the party of Dwarves who traveled to the Lonely Mountain in order to reclaim the Dwarven city of Erebor.  His alterior motive was to prevent Sauron from taking control of Smaug just as his master, Morgoth, had controlled Ancalagon and Glaurung.

"Smaug had left his lair in silent stealth, quietly soared into the air, then floated heavy and slow in the dark like a monstrous crow, down the wind towards the west of the Mountain, in the hopes of catching unawares something or somebody there, and of spying the outlet to the passage which the thief had used.  This was the outburst of his wrath when he could find nobody and see nothing, even where he guessed the outlet must actually be." ~The Hobbit, chap. XII (Inside Information)