T6 are generally good, sometimes T5’s, too but check the forums posts when an exo comes out to be sure.
Echoing what has been said, but on the permanents, probably get most from Houchus sapphire runs. Gem mining on Norkyna is more profitable for me overall, but not as many fossils. Exos can be great, even just picking through the areas tunneled through by others, many seem to ignore them. They pay for some of my trips.