What temperature do you foresee, if NaK is volatile?
The universal efficient cooling liquid is water. It has a huge capacity, and its small viscosity permits a finely turbulent flow that makes a good thermal contact with the solids.
Users commonly add some antifreeze like propane diol, some wetting agents, defoamers, anticorrosion, possibly antifungals... but basically it's water. About every other liquid is much worse, so it would need excellent reasons to search for a different one.