Between these 3 steels, which PM2 would reign supreme? Rex45, K390, 4v Rules: Hardness would be indexed to tested production hardness of the existing Spyderco PM2's exclusively. Edge geometry considerations can be maximized to post-factory sharpening. Corrosion is not a factor as blades are coated in this imaginary scenario. Non-answers like "not noticeably different", "would be better in a fixed blade", and comparisons to other steels not listed in the question are disqualified. This is a head-to-head-to head battle for which would be the overall champion with your rationale explained.