The concept of separation of powers in government is most greatly influenced by C. Montesquieu. In his work "The Spirit of the Laws," Montesquieu outlined the idea that the best way to prevent tyranny and protect liberty is to separate government powers into different branches, each with its own functions and responsibilities. This idea significantly impacted the structure of many modern democratic governments, including that of the United States.
While John Locke's ideas about government and natural rights also contributed to political thought, it was Montesquieu who specifically articulated the separation of powers concept.