Bilbo, alone and armed with an Elvish dagger and a magic ring, versus a cluster of giant spiders in the darkest part of Mirkwood Forest ... and the spiders never stood a chance! They even become terrified of the newly christened Sting! (Plays out similarly in the animated movie.)
Bilbo managing to evade the notice of the Mirkwood elves for weeks while successfully finding a way for him and his companions to escape.
Bilbo walks alone into a dragon's lair. A dragon, mind you, that managed to destroy two great kingdoms and their armies in a single day. Alone and with only a magic ring, Bilbo goes into Smaug's lair not just once, but twice and manages to hold his own in speaking with the beast who turns out to be cunning as well as powerful!
The Battle of the Five Armies, with the Humans, the Elves, and the Dwarves in a hastily forged alliance against the Orcs and the Wargs. And just when everything seems lost, Bilbo sees something on the horizon. What he says next serves as a very short Rousing Speech and a Battle Cry for the gathered forces.
First he raises the alarm when he realises that Smaug is coming, so that Laketown has time to prepare itself, then keeps encouraging the soldiers and archers not to lose hope as they try to bring Smaug down.
And then, when everyone else is fleeing, he's still firing arrow after arrow as the flames draw nearer, refusing to give up. When he learns the secret about Smaug's weakness he makes the million-in-one shot that finally kills Smaug, quite likely knowing that he'll probably die as well when the dragon collapses on the town; he was fully ready and willing to give his own life to save his people. Originally he was meant to die, but Tolkien himself recognised how awesome Bard is and kept him alive for the finale.
Smaug's assault on Erebor and Dale is every bit as glorious as it is horrifying. He singlehandedly razed not one, but TWO of the most powerful kingdoms in Middle Earth within an afternoon. Sauron himself needed a huge army and years of planning to attempt to overthrow Gondor... but who needs countless minions and intricate plans when you are Smaug the Magnificent?
If you know his backstory, it becomes a very dark sort of awesome. He was one of Morgoth's Fire Drakes, creatures who fought alongside Balrogs as shock troops for Sauron's boss - "I laid low the warriors of old..." Destroying whole kingdoms was his job, and he was happy to keep doing it long after Morgoth had left the picture.
Furthermore, it's implied that he only assaulted Erebor because King Thrór amassed an obscene amount of wealth; the only reason he didn't do so sooner was because he didn't deign it to be worth his time. He literally destroyed the greatest dwarven nation in the world on a whim.
Smaug: I kill where I wish and none dare resist. I laid low the warriors of old and their like is not in the world today. Then I was but young and tender. Now I am old and strong, strong, strong!