In a fight between King Kong and Godzilla, I'd think Godzilla would win.....easily. It's not a matter of favourites, I'm just looking at the defensive capabilities of each animal.
King Kong is covered in fur, which is potentially flammable. Underneath that fur is just skin like any other mammal, albeit thicker at that size, but probably more prone to puncture than Godzilla's reptilian based skin.
Even if Godzilla decides to play fair and not torch King Kong from a kilometer away, it seems likely that Godzilla should be able to fairly easily chomp through Kong Kong's fleshy skin. Just imagine a large salt water croc versus a silverback.
Of course, this being a movie, they'll probably end the movie on a draw or have the two team up against another foe to ensure they don't piss off either fan base.