Use the noun sultan when you talk about the king of a Muslim country. It's most commonly used to refer to the sultans of Turkey's Ottoman Empire.
There are some countries today that still use the term sultan for a ruler or nobleman, including Oman and Malaysia. However, the word comes up most often in a historical context, especially when you're talking about the former Ottoman Empire, where the title of sultan was inherited, passed from father to son. The word sultan has many different meanings in Arabic, including "strength," "ruler," "king," "queen," and "power."