I don’t know exactly what it is about this word, but it’s one of my favorites.
diadem (n.) IPA Pronunciation:
A cutpurse of the empire and the rule,
That from a shelf the precious diadem stole,
And put it in his pocket!
– Hamlet (III.iv)
The word just has such a regal sound about it. Crown sounds authoritative, but diadem is royal.