If you mean metal strength levels then obviously the hardest is Adamantium and, kind of, Uru (or at least the kind that's in Thor's hammer, as it's been able to dent Cap's Shield - and that's arguably the second best metal in 616), followed by Cap's Shield.
However, carbonium (a byproduct of adamantium production, or a different thing entirely, or a 'sister' metal to adamantium - depending on your writer) is able to destabilize adamantium, which makes it pretty strong, and, apparently, good for bullets (Bucky uses them on Daken at least once) and Magneto apparently has some pretty strong metals that he uses from time to time on his bases, as they seem to keep people down pretty easily.
Also, Colossus' 'metal' skin is a super strong steel like product called osmium, which is super dense and way stronger than most anything else in the comic universe, as he's been seen taking hits from Wolverine with no problem before.
If you mean mettle as in the D&D ability, in which a character's determination allows them to shrug off magical effects that would otherwise harm them by allowing for, when they make a successfully Will or Fortitude save, a negation of the effects, then... well... why are you asking here and not a D&D Forum?
Hope you meant metal, and that this answer helps.