4. Gellert Grindelwald
Like a psycho East European Dumbledore, Gellert Grindelwald was the most powerful dark wizard known before the rise of Voldemort. He became close friends with Dumbledore when they both lived in Godric's Hollow and their friendship would shape both their lives. But Grindelwald's ambition was too powerful and led to the accidental death of Dumbledore's sister, Ariana. Grindelwald went on to lead a violent wizard revolution, but was eventually defeated by forces led by Dumbledore and locked in his fortress, Nurmengard. When Voldemort's search for the Elder Wand led him to an elderly Grindelwald still stuck in his fortress, Grindelwald was still able to use his powers of occlumency to prevent the Dark Lord from peering into his mind.