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.
Minerva McGonagall
McGonagall is a master at Transfiguration, and her ability to duel is on full display during the last film when the battle on school grounds is taking place. On top of this, she is distinguished for her leadership and courage, as well as her strength in defending her students.
Minerva McGonagall
A total badass. She's the Head of Gryffindor and the most powerful witch in Hogwarts.
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.
Dumbledore. McGonagall was no slouch, and was a student of Dumbledore himself, but she wasn't quite on his level in terms of power and skill. McGonagall needed the help of 2 other very powerful wizards (Kingsley and slughorn) to fight Lord Voldemort and all three still lost against 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.
Strong: Albus Dumbledore
Most regarded him as the strongest wizard in history, and even Lord Voldemort himself feared to face him. Beyond his many impressive titles and positions of power, Dumbledore exhibited absolutely extraordinary abilities on several occasions.
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.
Minerva McGonagall
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.
Professor McGonagall is a Death Eater
According to the theory, McGonagall's seeming disregard for Harry Potter's safety, her talent for transfiguration, and and her stoic nature all point to the fact that she was a Death Eater.
Headmaster of Hogwarts throughout most of the Harry Potter series, it is safe to say that Dumbledore is the most powerful professor at Hogwarts. With his Elder wand, Dumbledore managed to get away both times he was threatened imprisonment at Azkaban. He could also duel Voldemort and hold his ground.
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.
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.
Riddle is 9 years OLDER than McGonagall.
She is someone Voldemort respects as she is a tremendously powerful witch.
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.
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.
She accuses them of concocting the whole dragon story to lure Malfoy out of bed and get him into trouble. As punishment, McGonagall deducts fifty points from Gryffindor for each of the three wrongdoers. Harry is horrified that his house will lose 150 points.
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.
Definitely McGonagall. She is much older and more experienced than Hermione. But she does not seem to have made good good use of her powers.
Lord Voldemort
Tom Marvolo Riddle started both Wizarding Wars, created the Death Eaters and killed Harry Potter's parents. We could go on – he was the most dangerous Dark wizard of all time – but we won't.
During book 7 is definitely Voldemort. By the time of the Cursed Child, it's probably Delphini. As for your other questions: Dumbledore was not powerful because of the Elder Wand. He took it from Grindelwald after defeating him in a duel (with a regular wand).
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.