I actually think Penance takes it. She's a quick little thing, and if she was determined to kill Wolvie, she could so some major internal damage with those hands of hers.
A little difficult to say no conventional weapon ever really hurt Penny and she carried out Wolvies "Sneak n' Slice" type tactics with near perfection
Im not sure about her doing much damage to his Bones but i dont think he could hurt her seriously enough to stop all the fleshy parts being ripped off his body in a prolonged fight [kinda gruesome actally].F
or her overall level of hardness Husk turned herself into Diamond when Penny was scard once and gave her a hug and it took only about a sec before Husk noted se was starting to cut into her hide
Of course ith her looks and bondage style fashion sense how does hairball stand a chance
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The only place it has been established that Wolverine can't cut Colossus was in Ultimate X-men. In 616 Wolverine was messing around with Colossus in the danger room and Wolverine swung at him and Colossus almost has a conniption, Colossus seems to think Wolverine can cut him.
Then there is a matter of his organic steel most likely being harder then diamond...
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Penance takes this. She's just too durable for Wolverine to hurt. Even if adamantium is harder than diamonds (I can't recall if it's defined as the hardest known substance or the hardest known metal) then he still needs to put adequate muscle behind it to actually hurt her. He still has fleshy parts, simply coming into physical contact with her causes damage, she could hug him to death if she wanted to.
Is Colossus' organic steel meant to be harder than diamonds? I forget.