Which of the anti-caking agents is best for boric acid base insecticide

In my experience the most effective boric acid insecticides are the granulated crystals, which are 99% pure. If you include form in your definition of "anti-caking," my answer would be granulation.

If you are referring to the 85 - 90% formulations of Orthoboric Acid, I have not found much difference between the various adulterants in terms of usage or effectiveness, so "best" could only be defined in terms of cost of manufacture, which varies.