Awesome question to which I have no universal answer for. For me, I distinguish a musical compositions melody first. I think this is why I like 'classic/acid rock' so much and hate rap. Rap plays monotonous beats over and over repeating the same monotone lyrics. I have a problem with modern so called R&B for similiar reasons. Just because a singer can hit various notes doesn't make them good. It depends on how they construct the notes. For me, well constructed melodies form the same structures as words in well written poetry. Again, just my opinion.