During times of war Gloom wolf fur rises a lot in popularity due to demand of death res imbue
In normal times piece of dead brain usually has the highest price per piece due to very limited availability as it only drops from braindeaths which only has very limited spawn as well and high demand as almost every knight uses life leech these days.
Demand is extremely high on any item used for Life leech, mana leech and critical strike. Most respectively those which are not hunted daily by a lot of people. as mentioned below/above > Sabreteeth, rope belts and protective charms but also vampire teeth and the above mentioned piece of dead brain.