Little is known about who is considered on this list. However, it can be assumed that some of these Dark Witches and Wizards are immensely powerful and dangerous, to a degree that rivals or equates to that of Voldemort or Grindelwald.
Gellert Grindelwald became the top of the list for the fanatical revolution he engineered against the European Ministries of Magic in his search for all three of the legendary Deathly Hallows, with which he intended to overthrow the International Statute of Wizarding Secrecy (thus exposing the entire Wizarding World to the outside world) and establish a benevolent hierarchal world order run by Wizarding supremacists, with Muggles made subservient to Witches and Wizards.
Born around 1883 in northern Germany, Grindelwald showed keen interest in the lore and powerful mystique of magical artefacts as a child, and was so fascinated with the Hallows that he adopted their runic symbols as his own personal emblem. He attended Durmstrang Institute in 1893 and became one of the most brilliant students the school ever saw, but after getting expelled in his sixth year for conducting dangerous experiments, he continued his education abroad. His gathered information on the Hallows eventually led Grindelwald to the British wizarding village of Godric's Hollow, the home of his historian great-aunt Bathilda Bagshot (the renowned author of A History of Magic). While there, the young wizard gained much power and followers terrorist activities against Muggle authority that were (in his own words) "For the Greater Good", making 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.
Tom Riddle became the top of the list, two and half decades after Grindelwald, for the two civil subversions he engineered against the British Ministry of Magic under the alias "Lord Voldemort", with which he intended to overthrow the Fundamental Laws of Magic and establish a malevolent tyrannical world order run by pure-blood supremacists with Muggles and Muggle-borns pressed in bondage and Voldemort himself declared the immortal ruler of all time. He was beyond more ruthless and cruel than Grindelwald ever was, being more of a tyrant than a misguided revolutionary, 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 from with top grades, considered to be by many and by Albus Dumbledore himself to be the most brilliant student Hogwarts has ever seen. Voldemort prefers to operate in secret as opposed to Grindelwald's open revolution, to sow the seeds of distrust and keep the oppressed confused, and having studied arcane and obscure magic after graduation while keeping a low profile and experimenting so ill-advisedly with the boundaries of magic to gain the power that transcend what others believe to be "normal evil". Voldemort's first and final downfall at the hands of Harry Potter made the latter a world famous hero.