Little is known about who is considered on this list. However, it can be assumed that some of these Dark Wizards
Gellert Grindelwald became the top of the list due to his revolution to overthrow the International Statute of Wizarding Secrecy and the European Ministries of Magic, which led to terrorist activities against Muggles that were, in his own words, "For the Greater Good". Born in 1883 Though considered to be one of the most brilliant students Durmstrang has ever seen, Grindelwald was expelled in his sixth year for conducting dangerous experiments. Grindelwald, being a gifted and powerful dark wizard, gained much power and followers that made it very difficult to stop him. He managed to create Nurmengard fortress to hold his prisoners and he caused much casualty, leaving him to be much feared, making his position as the #1 most dangerous dark wizard well deserved. His defeat at the hands of Albus Dumbledore was noted to be the greatest wizarding duel of all time, and Dumbledore became world renowned for his feat.
However, one generation later, after Grindelwald was defeated by Dumbledore, the former Dark Lord's position as the top of the list was taken by Lord Voldemort, who was beyond more ruthless and cruel than Grindelwald ever was, being more of a tyrant than a misguided revolutionary. Having engineered two wars to dominate the Wizarding World, creating an army of followers and deadly magical creatures that caused more fear and causalities than the previous conflict, and actually temporarily succeeded in controlling the Ministry. Voldemort was feared so much that the majority of the wizarding world would not even speak his name, flinching if heard, and instead referring him as "You-Know-Who" and the likes. Many believe that Voldemort was not human enough to die after his first downfall, furthering his superiority as a dangerous dark wizard, as unlike Grindelwald, he was unrepentant. Voldemort managed to keep his activities hidden and graduated with top grades, considered to be by many and by Albus Dumbledore himself to be the most brilliant student Hogwarts has ever seen. Voldemort's first and final downfall at the hands of Harry Potter made the latter a world famous hero.