1. Albus Percival Woolfric Brian Dumbledore
"The only person he was afraid of."
The dark lord who has ever been the most powerful man is afraid to confront him head-on, which is a good proof.
Dumbledore's magical knowledge and power was second to none. Before becoming Headmaster of Hogwarts School of Witchcraft and Wizardry, he served as a Transfiguration teacher for many years, and the only reason that prevented Dumbledore from gaining powerful positions such as Minister of Magic was his love of teaching rather than politics.
Dumbledore proved his magical expertise on numerous occasions, demonstrating advanced transfiguration and elegant dueling art during the Ministry of Magic's duel with Voldemort.
2. Tom Marvolo Riddle (Voldemort)
The most powerful Dark Lord in history.
His strength is unbeatable. He created seven Horcruxes for the first time in history, and is one of the few characters who can fly without a broom.
Voldemort was able to wrestle with Dumbledore for a long time without falling behind (arguably at his disadvantage, but still between Himon), and single-handedly played against Minerva McGonagall, Kingsley Shacklel, and Horace Slughorn (the three most accomplished wizards of his time).
3. Gellert Grindelwald
Grindelwald's feat of nearly destroying Paris with Demonic Fire is arguably the most impressive feat in the Harry Potter cinematic universe to date. And, as we all know, he later proved himself to be strong enough to challenge Dumbledore in a duel (although the exact course of the battle remains a mystery).
4. Severus Snape
The world's most powerful master of brain occlusion, he is very proficient in defence against the dark arts and potions.
Snape is a highly skilled wizard with the ability to create spells and potions himself, with unique flying abilities.
He defeated Harry Potter with relative ease and was able to retreat during a battle with Professor McGonagall.
5. Minerva McGonagall
Minerva was one of the most powerful witches of her time. She is very powerful when she transforms, and she is also an Animagus.
When she confronted Snape and later Voldemort himself, she proved her dueling skills, despite the help of Kingsley and Slughorn.
6. Filius Flitwick
Former dueling champion became teacher. He was able to defeat the Death Eater Antonin Dolokhov and create a super-powerful armor talisman in the final battle.
7. Bellatrix Lestrange
Voldemort's minions were the most loyal and cruel.
She was the one who killed Sirius Black, who was himself a famous wizard. Before being killed by Molly Weasley, she duels with Luna, Ginny, and Hermione at the same time.
8. Antonin Dolokhov
Dolokhov was another highly skilled Death Eater on Voldemort's ranks.
Dolokhov defeats moody the mad-eyed man in the Battle of the Department of Mysteries, causing some difficulties for the trio, killing Lupin and engaging in a fierce duel with Flitwick before falling.
9. Alastor "Mad-Eyed Man" Moody
As a battle-hardened former Auror and member of the Order of the Phoenix, Moody captured many Dark Wizards. His magic eye can see through any disguise, which gives him an advantage in battle.
Among the squad escorting Harry Potter, Moody was considered the greatest threat and therefore a target for Voldemort himself.
10. Kingsley Shackler
Another mighty Auror. Kingsley proved to be enough to duel multiple Death Eaters at once and stand out. With the help of McGonagall and Slughorn, he was able to duel Voldemort.