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 may closely rivals 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 fabled 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, but was so fascinated with the Hallows' legend in the The Tales of Beedle the Bard to the point of adopting their runic symbol as his own personal emblem. He attended Durmstrang Institute for Magical Learning in 1893, where he excelled as a precociously brilliant student, but was expelled at the end of his sixth year for conducting twisted experiments and thus continued his own magical education abroad.
In the summer of 1899, his historian great-aunt Bathilda Bagshot (the renowned author of A History of Magic) invited him to her home in Godric's Hollow, the British wizarding village which served as the burial place of the three brothers who originally owned the Hallows. During his stay, she introduced him to her brilliant yet like-minded young neighbour Albus Dumbledore, who was drawn to Grindelwald's ideas of greater power. 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 was forced to flee, 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. With the Elder Wand augmenting his already immense power, Grindelwald seemed to be unstoppable, with many who fought him being defeated and killed. As he continued to amass more power, public quickly began to plead for Dumbledore to fight him as he was the only one who could possibly be Grindelwald's superior. Although Dumbledore tried his best to resist it due to his fear of Grindelwald knowing who killed Ariana, eventually Grindelwald proved too powerful and dangerous and Dumbledore was finally forced to combat him in a duel so legendary it 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, dying trying to protect the Elder Wand from him as an attempt to achieve a degree of redemption.
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 sinister, 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. Even towards his own subordinates he has very little mercy or care for them, as he would easily kill or tortuously punish them for the slightest signs of failure or disloyalty, or even for his own self-serving purposes; for Voldemort, his own life and status was the only thing he ever values, and that everyone else was just a stepping stone towards his own goals. 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. Indeed, even Dumbledore acknowledged him as the most dangerous dark wizard of all time, as not only was he a far worse dark wizard than Grindelwald, he at least rivalled Grindelwald's might, if not surpassed him, although whether he could match Grindelwald wielding the Elder Wand remains unknown, and indeed the only known individual to ever overwhelm him was Dumbledore himself wielding the Elder Wand and even Dumbledore could not truly defeat him, albeit Dumbledore had held back as he never intended to kill Voldemort anyway.