General Synonyms:
* Monarch
* Sovereign
* Ruler
* Regent
* Head of State
* Emperor (for a king who rules over an empire)
* Chieftain (for a leader of a tribe or clan)
Formal Synonyms:
* His Majesty
* Your Majesty
* Sire (an archaic and respectful term)
Informal Synonyms:
* Boss (in a casual setting)
* Bigwig (for a powerful and important person)
* Head honcho (slang for the person in charge)
Figurative Synonyms:
* Lion (a symbol of strength and power)
* Lord (a powerful and respected figure)
The best synonym for "king" will depend on the specific context and the level of formality you want to achieve.