Some years ago i've compared most of the microwave laminates.
Generally speaking all laminates are low loss material. Probably Rogers 5880 is the lowest loss.
But take in account some pratical considerations.
60 Ghz circuit may be very small, and 5880 is one of the softest material. It tend to be similar to the plaster and the copper line tend to "float" over the dielectric . This reflect on the mechanical accuracy.
Using a rigid material, you can get higher mechanical accuracy, but often they have larger er.
Try PFTE loaded with fiberglass (Taconic TLY...).
Useful info may got on Laverghetta-Stripline circuit Design.