* Monarchy: A form of government where a single person, like a king or queen, holds supreme power.
* Dictatorship: A form of government where a single person or a small group has absolute power, usually gained through force.
* Empire: A large, powerful state or group of states ruled by a single person or government.
* Totalitarian state: A state that controls all aspects of its citizens' lives, leaving them with no freedom.
* Autocracy: A form of government where one person has unlimited power.
* Tyranny: A cruel and oppressive government or rule.
The best antonym for "republic" will depend on the specific context in which the word is used.