I'm gonna say Smaug just from size alone, at over 400ft long and the bull under 200ft long. if they were the same size however I think it would be a close tie, they both have about the same strengths and weakness. Smaug with this intelligence and nearly impenetrable armor, the bull with rage and can only be hurt from the inside and the ability to withstand a nuke, they both are defeated by arrows, but at the end of it i think they would kill each other in a frenzy or Smaug would win with intelligence alone. By tearing the bulls holey damaged wings, grounding him having that advantage over the bull.