I'm going to go with Durin's Bane/Balrog. Reason one: I will admit it's preference on my part. Reason two: they are essentially spirits who helped create the world. Reason three: Smaug was taken out by a single arrow (yes it was a small area to hit, but regardless it's a weak spot). Reason 4: it took a lot out of Gandalf to beat the Balrog. However, he willingly went with the dwarves to go reclaim the Lonely Mountain, so he must have been confident they could beat Smaug or he wouldn't have bothered to help.