which substrate is best for design high freq microstrip line

The tradeoff is that low er usually has lower losses but the circuits are larger.

Generally, the loss is a sum of 2 contributes, namely conductor loss and dielectric loss. Usually the 1st il larger than the latter.
Using low er dilectric, the metal is geometrically larger, so less lossy.
But conductor loss is dependent on dimensions, conductivity and roughness.
Whit hard substrates (lile Alumina, Quartz, etc) it's possible get lowest roughness.
Unfotunately hard substrate have high er.

So for microwave frequencies there is no big difference, for millimetric frequencies the best materials are low er or low roughness materials, but not flexible PCB (like Rogers 6010) because it's higher roughness.

Alumina 96% Al2O3 is very good for your application