General Synonyms:
* Monarch
* Sovereign
* Ruler
* Head of state
* Regent
* Emperor (for a king who rules an empire)
* Chieftain (for a leader of a tribe or clan)
* Lord (more formal and archaic)
Figurative Synonyms:
* Master (implies control)
* Boss (informal)
* Chief (implies leadership)
* Tyrant (implies cruelty)
* Potentate (a powerful ruler)
Specific Contexts:
* Chess: The king piece in chess is often called the monarch.
* Playing Cards: The "King" card is sometimes referred to as the royal.
* Nature: The lion is often called the "king of the jungle".
The best synonym for "king" will depend on the specific situation and what you want to emphasize.