What is the Tagalog word of honesty best policy?

There isn't a direct, idiomatic translation of "honesty is the best policy" in Tagalog. However, you can express the same idea in a few ways:

Literal Translation:

* Ang katapatan ay ang pinakamagandang patakaran. (Ang katapatan - honesty, pinakamagandang - best, patakaran - policy)

More idiomatic:

* Mas mabuting maging matapat kaysa magsinungaling. (Mas mabuting - better, maging matapat - be honest, kaysa - than, magsinungaling - lie)

* Ang pagiging matapat ang magdadala sa iyo sa tagumpay. (Pagiging matapat - being honest, magdadala - will bring, tagumpay - success)

* Mas maigi ang katotohanan kaysa sa kasinungalingan. (Mas maigi - better, katotohanan - truth, kasinungalingan - lie)

The choice of which translation to use will depend on the context and the intended meaning.