Albus Dumbledore is identified as Hogwarts’s longest-serving headmaster in J.K. Rowling’s Harry Potter story.
Dumbledore serves as headmaster for most of Harry Potter’s time at Hogwarts School of Witchcraft and Wizardry. He is a powerful wizard, a mentor to Harry and a major opponent of Lord Voldemort. His leadership is central to the series because he helps prepare Harry and others for the final conflict.
The books also establish that Dumbledore was preceded as headmaster by Armando Dippet. Dippet was headmaster when Tom Riddle, later Lord Voldemort, attended Hogwarts. After Dumbledore’s death, the school’s leadership changes again, including a brief period under Severus Snape.
The wording matters because “most famous” or “most powerful” would be subjective, whereas “longest-serving” is a stated biographical fact in the wizarding-world material. Dumbledore’s tenure is also distinct from his earlier career as a Hogwarts Transfiguration professor.