PCB material for high-speed designsm

Can you recommend a few materials that provide low leakage loss on boards operating above 20 GHz?

Leakage loss becomes increasingly crucial as you operate at high frequencies. For boards operating above 20 GHz, you may want to consider materials that have high volume resistance, which results in minimal leakage losses.
Here are some low loss materials that you can consider:
EMC’s EM-528, EM-890K, Panasonic’s Megtron 6, Isola’s I-Tera MT40, Astra MT77, Roger’s RO4350B, RO3003 for RF applications

EM-890K, Megtron7, and Rogers rt Duroid are good choices, the last being most likely the best at those frequencies.