Yeah, you can
Insult me multiple times and then post a picture saying "come at me bro" but you're not an internet tough guy. No, not at all.
While heaviness may be the opinion of the individual, I am under no obligation to agree, or even respect said opinion. The standards which we define musical terms by are subjective, but there are more logical paths that are used to define those standards than others.
I could say old school rap is the heaviest genre of music because it has harsh lyrics and
MAD BEATZ, and if you were to disagree with me, I could say, as you put it, "YOU JUST CAN'T TELL." You're essentially following that path of logic, simply on a smaller scale.
Oh, and you can sure as fuck compare two different subgenres because the thread was asking what the heaviest METAL band was, therefore you can incorporate any bands from the metal spectrum in your discussion.
EDIT: shit, I almost forgot.
hypocrite.
If you are a false, don't entry, or you'll be burned and died!