Let's say that instea of moria, the balrog was sleeping beneath Erebor. When Smaug occupied Erebor, he inadvertently awoke the Balrog from its slumber and immediately attacks Smaug. Who will come out on too.
Don't know about the book version, but based on the movies, I am open to be corrected but leaning more toward Balrog.
After all they are both fire-based creatures, which means they should be immune to each others' fire attack. But unlike Smaug, apart from flame, Balrog also have giant sword and whip. Smaug can fly around all he wants and spray fire at Balrog but it's not gonna do anything, but the moment Balrog tangle him up with his whip, he could attempt to pierce him with the sword.
Yes, movie Smaug's hide is incredible tough but it's not 100% invincible, so it's possible that it could be hurt by the ancient creature's giant blade.
Unless of course, my memory deceived me, is Smaug several times bigger than Balrog and can use him as a chew-toy? if that is the case then yeah, Smaug got it. I haven't bother to check their official sizes/stats lol