My son asked me this very tricky question and I'd be interested to hear opinions: Who would win in a fight between the Balrog from Moria and Smaug the dragon?
Things to consider; Smaug, despite being very powerful, is nevetheless a creature of flesh and blood and is susceptible to physical damage. The Balrog is a spiritual creature, but can be killed (although Gandalf presumably had to use magic to achieve this). So it's a tough call but it would be some fight...