Albus Percival Wulfric Brian Dumbledore was the only wizard Voldemort ever feared and the only one who could have defeated him in a duel. Albus Dumbledore is the most powerful wizard in the Harry Potter.
In the Harry Potter franchise, the two most powerful dark wizards of all time only briefly came into contact: Gellert Grindelwald and Lord Voldemort. Grindelwald was older and didn't stand a chance against Voldemort.
Gellert Grindelwald
After falling out with Dumbledore, he became the master of the most powerful Elder Wand and led a years-long crusade against the Wizarding World.
4 Merlin. Documents in the wizarding world suggest that Merlin was one of the most powerful wizards in Harry Potter and human history in general. Merlin was a Slytherin at Hogwarts and demonstrated an amazing understanding of the magical arts. Some argue him to be the most powerful wizard of all time.
[Harry Potter] Voldemort was more powerful than Dumbledore; Dumbledore was more powerful than Grindlewald. In some of the backstory of the Harry Potter books we learn that Dumbledore acquired the Elder Wand by defeating Grindlewald in a duel.
Albus Dumbledore is the most obvious person who can defeat Voldemort, and the Dark Lord seemed to realize it. Rowling herself has explicitly stated in the books that Dumbledore is the only person Voldemort fears and recognizes as a genuine threat to his own power.
Additionally, Snape is more skilled in potions than Dumbledore, making him a powerful contender for Albus's spot on the list. Albus, however, edges out Snape thanks to the former's greater base power, and deeper understanding of the complexities of “old” magic.
Albus Percival Wulfric Brian Dumbledore was the only wizard Voldemort ever feared and the only one who could have defeated him in a duel. Albus Dumbledore is the most powerful wizard in the Harry Potter.
Godrick Gryffindor, one of the four founders of Hogwarts, and the owner of the legendary Sword of Gryffindor, was absolutely more powerful than Dumbledore— and it's a good thing he was, too, because Dumbledore wasn't around in Hogwarts' early days to keep Godrick's fellow founder Salazar Slytherin in check.
For example, in Harry Potter and the Deathly Hallows – Part 2, Snape was reluctant to fight McGonagall when the latter took him to a duel in front of Hogwarts students. Both characters had their strengths, but the last Harry Potter movie proved Snape was the strongest Hogwarts professor.
Why was Draco the true owner of the Elder Wand? Voldemort thinks that he becomes the true owner of the Elder Wand by stealing it from Dumbledore's grave, but in the end we learn that the true owner was really Draco Malfoy, that is until Harry defeated him and allegiance transferred to Harry.
Following Lord Voldemort's defeat at the hands of her student Harry Potter and the deaths of Headmasters Albus Dumbledore and Severus Snape, McGonagall takes the position of Headmistress.
Avada Kedavra
Topping the list for the most powerful spells in the Harry Potter universe is no other than Avada Kedavra, Voldemort's favorite to instill his will and fear onto others. The spell itself is a play on Abra Kadabra and is easy to cast by just about anyone in the Wizarding World.
Dolores Umbridge is in a position of power and is allowed to stay in it even after abusing it. Lord Voldemort, on the other hand, is evil, whereas Umbridge believes she is one of the good ones and that her actions are justifiable, even necessary.
There were a few members of the Death Eaters that Voldemort valued above others. Of course, Bellatrix and Snape were easily his favorites, but the Carrow siblings were up there as well. There are many instances that prove that Voldemort appreciated Amycus and Alecto Carrow.
He did not love anyone, and did not care to be loved. He felt nothing for his family, and only wanted his friends in school to rally around him because he wanted to hold power over them. Bellatrix Lestrange was obviously in love with him and worshiped him, but he only cared for her servitude, not her.
Like Tom Marvolo Riddle (aka Voldemort), Merlin was a Slytherin, an ambitious, powerful wizard. He was a wizarding legend, and, of course, more powerful than the Dark Lord. According to the Harry Potter Wiki, Merlin was “arguably one of the most powerful wizards of all time”.
In his prime, Dumbledore truly had no equal who could best him in a one-on-one wizarding duel. Even though he might seem like an old man, his wizarding power is on a completely different level. Gandalf is probably the most powerful opponent he'd ever face, but Dumbledore wouldn't go down without a fight.
While Dumbledore is undoubtedly a powerful wizard, Gandalf's innate magical abilities give him an edge in a battle between the two. Gandalf's fire magic, telekinesis, and healing powers would make him a formidable opponent for Dumbledore.
The Longbottoms
Neville comes from a long-line of pure-blood wizards, although we don't know as much about the individuals as we do with some other wizarding families. What we do know is that both of Neville's parents were pure-blood, as was his gran, Augusta.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
All seven novels show Snape to be a very powerful wizard and to have been outstanding while a student. He specialises in potion making and has talent and passion for the Dark Arts. Sirius claimed that Snape knew more hexes and curses as a first-year student at Hogwarts than most seventh-years knew.
In the film, Snape is forced to fight Professor McGonagall after Harry Potter reveals that he was the one that killed Professor Dumbledore (at Dumbledore's bidding, though at the time the fact was unknown).
Bellatrix would be in the top pile of witches and wizards of all-time based on powers, but her reliance on the same moves and lack of wit means she would be defeated by Snape after giving him an initially tricky time.