Haha most of these things I can understand. However, since these two are from the same mythology, the same world, it is hard for me to fathom Glaurung (who Tolkien clearly portrayed to be stronger) losing to Smaug.
On another note, as much as some people may want Harry Potter creatures to lose this, it seems very unlikely to me that a spider kills a dragon under any circumstances. Shelob would get killed there.
Glaurung was definitely one of the strongest dragons, but I think the very strongest one was Ancalagon the Black, who was slain by Elrond Half-elven's father Earendil during the War of Wrath at the end of the First Age. I could be wrong though, it's been a while since I read the Silmarillion.
It's a case, once again, of Smaug being the "prettiest belle of the ball" like Gandalf and Aragorn were in the previous challenges. People know them, so they get more votes. We should just do a Lord of the Rings Death Match and let them all kill each other.
Ancalagon was the strongest (and, yes, was slain by Eärendil); Glaurung was the first… and no, I can't see him losing to Smaug, either.
While Shelob may or may not be able to best a dragon under the best of circumstances, her mom could've wiped out just about any of them, at least by the end of her appearance: she nearly offed Melkor, and was only driven off—and only driven off—by him summoning his balrogs to rescue him.