Smaug wasn't the first dragon I knew by name - that would have been Tiamat, the many-headed antagonist from the classic and awesome D&D cartoon. But Smaug was the first to carry a whisper of dread, to convey a tingling sense of malice and mystery and power. Smaug lurks in the third act of The Hobbit like a malevolent gathering darkness, and Tolkien builds the tempo of the narrative slowly, using other, lesser perils to set the stage for the truly terrifying beast inhabiting the Lonely Mountain and the nightmarish cultural memory of the dwarves.
When Bilbo finally penetrates Smaug's lair, there is no disappointment. Smaug delivers on all the menacing foreshadowing, a monster of such brute strength and fiery danger that the reader quakes right along with the poor hobbit. And yet Smaug also offers a kind of urbane charisma, marrying his overpowering physical presence with profound intellect and gravitas. His badinage with Bilbo is great writing and reading, some of Tolkien's most nimble and realistic dialogue, lacking some of the stentorian heaviness that characterized his later LOTR work. Some of this is that The Hobbit is lighter fare. Some of it is that Smaug is just awesome, in every sense of the word.
I'm a huge fan of the 1977 animated version of The Hobbit, released as a TV movie. The film was nominated for a a Hugo Award that year, but lost to this little picture called Star Wars. The Smaug in this rendition is perfect - suave, prideful, gone somewhat cocky and slovenly and yet still filled with latent hot rage and coiled destructive capability. Richard Boone's voice is perfect, alternating between lazy dismissal, gnawing suspicion, and explosive wrath. In fact, this version is so iconic, so definitive, that even an actor regarded by many as one of the finest around these days, Benedict Cumberbatch, was reduced to imitation in his voice performance in Peter Jackson's lamentable Hobbit fiasco.
And remember...his armor was like tenfold shields; his teeth like swords; his claws, spears; the shock of his tail, a thunderbolt; his wings, a hurricane; and his breath... death!