In the Harry Potter series, had Minerva McGonagall and Severus Snape both gone all out in a duel, who would have won? Severus Snape would win hands down, as evidenced by the clash of the seventh book or the eighth film.
In fact, Snape fought off McGonagall and directed her attacks against the Deatheaters behind him. Her desire to protect her students and her love of Hogwarts was on full display here. If at full capacity, defending everything she held dear, the new Headmistress McGonagall was unable to defeat Snape.
Snape still considered McGonagall a friend. More importantly, he knew McGonagall would be vital to protecting Hogwarts and the students from the upcoming battle with Voldemort. Snape knew he couldn't hurt her, but he couldn't just LET her win, though.
McGonagall was among the oldest and most experienced teachers in Hogwarts, and she was a worthy successor to Dumbledore as the headmistress. In fact, she has proven her mettle against Voldemort and survived a duel against him.
In the book, Minerva McGonagall is the one who does it and for me it was very important that she did that.” — J.K. Rowling, on fighting the decision to pit Harry Potter against Severus Snape in Harry Potter and the Deathly Hallows: Part 2 instead of McGonagall, who fought Snape in the book.
Later in the year, McGonagall duelled with Death Eater Alecto Carrow during the Battle in the Astronomy Tower and later learned of Snape's murder of Dumbledore. McGonagall was initially appointed as Headmistress, but was demoted after Voldemort took control of the school and placed Snape as Headmaster.
When McGonagall calls Snape a coward, she is not referring to his skills. She is in fact attacking his ethics and morals. From Harry and Co.'s perspective, Dumbledore was weak and alone and begging Snape to not kill him.
She was strong enough to kill Tonks (an auror). McGonagall had no such lack of discipline and was incredibly powerful in her own right. Bella was the only death eater that was able to deflect one of the spells from Dumbledore.
Bellatrix is (kind of) in her prime, while McGonagall is old. We should also know that although Bellatrix is evil, it doesn't mean she's stupid. But seeing that McGonagall is old, we can correctly assume that she's experienced. I'd say McGonagall wins the duel, no questions asked.
Weaknesses. McGonagall is never portrayed as having any particular weaknesses. Her stern facade likely prevents anyone from getting close to her emotionally, with the possible exception of Dumbledore. We are led to believe that she prefers her solitude.
Outnumbered, Snape dashes into a deserted classroom where a loud crash is heard. Pursuing, McGonagall screams, "Coward!" Uncloaked, Harry and Luna rush inside, startling the other professors. Snape has leaped out the window.
Snape hated Neville because he was a daily reminder Lily Potter could have lived. Snape's guilt over his part in her being targeted by Voldemort in the first place was bad enough, but to see a living, breathing reminder that things could have played out differently even after that act made Snape particularly vicious.
Snape questioned McGonagall as to why she was patrolling the corridors at that hour, and then discreetly used Legilimency on the Professor. As he asked McGonagall if she had seen Harry Potter, McGonagall quickly slashed her wand through the air, but Snape cast a Shield Charm that threw McGonagall off balance.
Winner: Snape
The kind of versatility in magical abilities that Snape displayed was almost around the level of Albus Dumbledore and Voldemort, making him easily more powerful than the average wizard and even the members of the Order of the Phoenix and the Death Eaters.
McGonagall followed up Harry's action by using the Imperius Curse on Amycus before tying him up with a net, rendering him useless during the Battle of Hogwarts.
Professor McGonagall did not know. It took her by surprise just as it took Harry by surprise.
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.
Minerva had mastered the Patronus Charm, being able to create a powerful corporeal Patronus in the form of a cat — identical to her Animagus — , being the only person seen to cast three Patronuses at once. She also was able to use her Patronus as a means of communication, as taught to her by Dumbledore.
Of the characters in the books, the top-tier wizards are pretty clearly shown to be Dumbledore, Voldemort, and Grindelwald. If one accepts that magic has advanced since ancient times, these three are probably even greater than the Hogwarts Founders themselves.
Dumbledore is one of the most powerful wizards to ever live. His exploits and achievements earned him fame across the Wizarding World and made him a top choice to be Minister for Magic, but Dumbledore choose to spend most of his life at Hogwarts, devoting himself to teaching future generations.
20 More Powerful: Minerva McGonagall
As I said earlier, many Hogwarts professors are arguably more powerful than Voldemort. Minerva McGonagall is, without question, one of them.
So Snape was cruel, biased towards his house, and downright rude to his students: this we can't deny. Calling poor Hermione an 'insufferable know-it-all', for example, was pretty terrible.
The Reddit thread agrees Snape's Boggart may revolve around the person who mattered most to him: Lily Potter. Snape's unrequited love for Lily lead to his greatest fear coming true, a fact that wasn't revealed until The Deathly Hallows.
Draco's refusal to reveal Harry's identity to Bellatrix was not because he liked him. It was because he believed that Harry was the only chance they had at defeating Voldemort. At first, Draco admired and revered Voldemort until he threatened his family's safety.