I'm not sure that a margraf or landgraf could be considered the equivalent of a duke. When the Emperor "upgraded" a margraf" it was to Fürst (prince) and not to Hertzog (duke).
What are the titles used by various reigning sovereigns? Emperor/Empress; King, Queen, Grand Duke/Duchess: Prince/Princess. Popes, too?
...but today in Europe, we only have kings and queens, a grand duke and grand duchess, and princes and princesses.
For sovereign continental titles in use at the end of WW1, you’d have the following relative ranking:
Emperor and empress
King and queen
Grand duke and grand duchess
Duke and duchess
Prince and princess
It gets a little more complicated when you add margraves, landgraves, etc, which I suppose might have had rank near or equivalent to dukes. Obviously, the ruling ones always outranked the noble ones and those who bore the titles as junior members of the family.
I think the Pope may technically be king of the Vatican state, or were temporally like kings (but don’t quote me on that!).