Strength, Hisoka clearly has this in the bag, transmution are closer to enhancement than specialist.
Chrollo nen power is better in combat, he can use nen powers he stole from other people, in hxh world that is OP. Hisoka nen powers bungee gum and tacture surpise are not that impressive, however they enforce his deadly, sneaky and brilliant mindgames. And thats how he won from Kastro who also had better nen powers. Hisoka fooled Chrollo hard during the Yorknew city arc, Hisoka can simply outsmart him.