Heroes 5.5 WarGame Edition - Tactical Edition.

bandits/robbers?

Robbers. But I wrote "raiders" in the poll, since it's possible to do so.

ruffians/stormtroopers/butchers/ogres?

Butchers > ogres > ruffians > stormtroopers

trolls/turs/jotuns/erls?

Turs.

lava/volcanic dragons?

I like "volcanic" more in terms of sound, but the final name is too long, I'm afraid.
Anyway, it doesn't really matter.

flaming priests/Flame veterans?

Flaming priests.

polar/arctic/snow priests/elders?

"Snow elders" – I like it.
Elders > priests (if the previous upgrade is priests, otherwise it doesn't matter)
Snow > polar >>> arctic (Arctic is completely irrelevant, it's earthly geography, not Ashan's)