"Dragons may not have much real use for all their wealth..."

but they know it to an ounce as a rule, especially after long possession; and Smaug was no exception." And, according to Forbes' Fictional 15, he is the richest, too...among fictional characters, that is. 

Benedict Cumberbatch will be the voice behind Smaug in The Hobbit. Photo from The-Hobbit Movie.

Here's the complete list of the richest fictional characters according to the magazine.