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 was number one 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 and himself declared the invincible dictator of all time.
Born in 1883, Grindelwald grew up showing keen interest in the lore and mystique of magical artefacts, and became so fascinated with the Hallows that he adopted their runic symbols as his own personal emblem. He attended Durmstrang Institute in 1893, where he proved to be an precociously brilliant student, but his twisted activities held too much ominosity for the school to tolerate and led to his expulsion by the end of his sixth year. His magical education by gathering 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 staying with her for the summer in 1899, he met and befriended her brilliant yet like-minded young neighbor Albus Dumbledore, who was inspired by his radical ideas. Together, they planned on rebelling against Muggle authority in brutal ways that were (in their own words) "For the Greater Good". When Dumbledore's younger brother Aberforth found out, he confronted the duo and attempted to interfere with the conspiracy, but Grindelwald retaliated by lashing out with the Cruciatus Curse. Dumbledore intervened, sparking a three-way duel in which the brothers' magically unstable younger sister Ariana was killed and Grindelwald fled, never to be welcomed to Godric's Hollow again.
He managed to create Nurmengard fortress to hold his prisoners and his revolution killed many, leaving him to be much feared, making his position as the #1 most dangerous dark wizard well deserved. His defeat at the hands of Dumbledore was noted to be the greatest wizarding duel of all time, and Dumbledore became world renowned for his feat. He was imprisoned in his own prison and lived there until he was murdered by a wizard more dangerous than himself, Lord Voldemort.
Tom Riddle is widely considered the most evil wizard of all time, 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 himself declared the immortal ruler of all time.
Born on 31 December, 1926, as the half-blood son of affluent Muggle squire Tom Riddle, Snr (whose family was the most unpopular land gentry in the Yorkshire village of Little Hangleton) and pure-blood vagabond witch Merope Gaunt (whose family was the one of the most heavily inbred and impoverish wizarding families among the Sacred Twenty-Eight).
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. His ruthless persecution of Muggles, his countless murders and terrorist atrocities, and the creation of multiple Horcruxes established him as the most evil wizard who had ever lived. Voldemort's crimes were considered so evil that his name became unspeakable. He 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 preferred 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.