Who is the main antagonist in the book that hobbit?

Smaug

I don't think the Hobbit has a continuous antagonist like say Sauron in Lord of the Rings. Instead there are a series of adventures and obstacles that can be considered antagonistic. These challenges offer have creatures that serve as antagonists too. Consider,

Trolls, Goblins, wolves, the Spiders of Mirkwood, the Wood-elves, Smaug and then multiple enemies in the Battle of Five Armies.