I'll vote for Kraton. Very grippy even when wet or bloody. Here is an example.
http://www.premiumknives.com/ShopSite/Ka_Bar_1258_Short_Utility_Knife_Black_Blade-Kydex_Sheath.html
http://www.premiumknives.com/ShopSite/Ka_Bar_1258_Short_Utility_Knife_Black_Blade-Kydex_Sheath.html