Little is known about who is considered on this list. However, Gellert Grindelwald became the top of the list due to his revolution to overthrow Muggles and the European Ministries of Magic, which led to terrorist activities that, in his own words, "For the Greater Good". 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 the Nurmengard prison 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 were far more ruthless and cruel than Grindelwald, 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. Another twist is that while Grindelwald was expelled for his dangerous experiments, Voldemort managed to keep his activities hidden and graduated with top grades. Voldemort's first and final downfall at the hands of Harry Potter made the latter a world famous hero.