Dumbledore aside, the debate on who Hogwarts' most powerful professor was in the Harry Potter series lies between Minerva McGonagall and Severus Snape. The series boasted an array of experts in the magic arts, with the professors at Hogwarts constituting a large majority of them. Dumbledore's position as the most powerful professor in Hogwarts is undeniable, but the extent to which McGonagall's impressive dueling skills can stack up to Snape's expertise in Legilimency is up to question.
In Harry Potter and the Sorcerer's Stone, it was clear the rivalry between McGonagall and Snape over Harry Potter was rooted in their mutual respect for Dumbledore. When McGonagall took Harry to Snape's quarters in Harry Potter and The Order of the Phoenix after a flurry of nightmares about Voldemort, it showed she trusted Snape's skills to aid Harry to overcome such nightmares. Though the extent to which she knew of Dumbledore and Snape's plot against Voldemort is debatable, their mutual respect for one another indicates the significant power they held as masters of the magic arts. 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. McGonagall's transfiguration reputation spoke for itself. She was able to morph herself into a feline, making for that comical moment in The Sorcerer's Stone when Ron thought he was safely in time for his lesson and could manipulate any animal into stationary objects. However, Harry Potter and the Half-Blood Prince proved Snape's ability to craft his own curses and alter potion recipes using his innate talent and magical knowledge. Though Harry could fight against his Legilimence spell in The Half-Blood Prince, it was because Harry, as the son of his lost love, Lily, was undoubtedly a weakness for Snape.
The duel between McGonagall and Snape proved the latter's status as Hogwarts' most powerful professor. Harry emerged from the crowd of students and confronted Snape for killing Dumbledore. Immediately as Snape took out his wand to fight Harry, McGonagall stepped in, making for one of the most interesting duels in the entirety of the franchise. At full power, Minerva cast fire spells against Snape, only for the latter to defend himself against her every attempt. 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 Heistress McGonagall was unable to defeat Snape. It suggested the latter was the more powerful professor.
Though Snape effectively brought the battle against Voldemort to Hogwarts by becoming the school's heaster and placing many lives at risk, his sacrifice was deemed necessary by him and Dumbledore to finally beat the wizarding world's arch-nemesis. Only dark wizards in the Harry Potter world can produce powerful attacking spells and craft their own curses. The only way Voldemort could be defeated was by playing his own greed against him and placing him in a direct battle with 'the chosen one.' Snape lost everything in the end, but his status as Hogwarts' most powerful professor was crystallized in his fight against McGonagall.
