What is the best insulator for suface resistivity? (and triboelectric charging?)
This table shows the best materials to use with high surface resistivity, the surface resistivity is more important than the volume resistivity because at high voltage (or low leakage currents) most of the current will take the lowest resistive pathway. 

Keep in mind that air has a much lower resistivity than these materials to guarding schemes will be needed or evacuation of air if these materials resistive properties are to be used. It's also important to maintain a clean surface as contamination can significantly lower the surface resistivity. 

Teflon has the highest volume resistivity but his hard to machine, poliyimide (kapton) is another great material to use. 


Source: https://www.researchgate.net/publication/265801794_Comparison_of_Surface_Resistivity_and_Triboelectric_Charge_Generation_Characteristics_of_Materials