In terms of order, I think the oldest monarch goes first; although I don’t whether that’s longest serving or chronological age; because when Charles becomes a King, he will be a good couple of decades older but a more recent king than his European counterparts.
Anyway reason being, King Hussein of Jordan said that at an event in the 50s, he was the youngest Monarch so last. Then at one towards the end of his life; he was the oldest; so first!