I'm pretty sure I've done this already, but will give it another go:
Queen Maxima - for her fun and vibrancy and to show us how to dance a tango!
Duchess of Cornwall - I'd like to know more about her, her personality etc
Empress Michiko - she can play the piano after the dinner
King of Bhutan - to tell us more about his mysterious country
Naturally they could bring their spouses along too!