Hogwarts contains some of the most powerful witches and wizards in the world. They are at the very top of their profession, but that doesn't always make them good teachers. While some are clearly incredibly intelligent and talented, it doesn't necessarily translate into their teaching.
At the same time, some of the professors that have been hired at Hogwarts, really shouldn't be working there at all. They are employed by circumstance, good will, or perhaps because dark forces are at play. These teachers are even worse than those who simply don't know how to pass on their knowledge. We're ranking 10 of the worst Hogwarts professors by their teaching ability.
Severus Snape
Snape is a well studied man with a vast knowledge of spells and magic. He is particularly talented with potions although he's always had his eye on the Defense Against the Dark Arts job. Nonetheless, he is in practice a very good wizard and can be relied upon to quickly create a useful concoction when called upon.
His teaching ability is poor at best though. He has very little patience for his students and certainly wouldn't take the time to explain something if they are confused. Students can rarely make mistakes in front of him and he seems more concerned with discipline than any actual mentoring.
Rubeus Hagrid
Hagird means well and has a very useful set of skills when it comes to the care of magical creatures. He is one with many of the animals and knows exactly how to treat them and defend against them if need be. But, he really isn't used to teaching this knowledge to a whole class of students and it shows.
He doesn't approach each lesson with health and safety in mind, even assigning books that could bite a hand off if the student isn't careful. Not only this, he allowed Harry to fly on a fully grown Hippogriff without supervision and even allowed the creature to attack another pupil... even if it was Draco.
Alastor Moody
The confusing thing about Alastor Moody is that the real Mad-Eye has never actually taught at Hogwarts School of Witchcraft and Wizardry. The Moody we are familiar with as the Defense Against the Dark Arts teacher, is actually Barty Crouch Jr using polyjuice potion.
Crouch is obviously a dark wizard with immense power and he doesn't have an issue with showing the students the unforgivable curses. He may know his curses and charms and isn't even too bad demonstrating them, but he's dangerously irresponsible and even turned a pupil into a weasel... ever if it was Draco again. Barty Crouch Jr probably shouldn't be let anywhere near children.
Madam Hooch
Madam Hooch is one of the best flyers around, although she struggles to teach the younger students how to pick up the ability for the first time. Showing them how to simply get the broom to fly to their hands has probably caused nose bleeds many times over the years.
Plus, she's allowed students to fly around unsupervised many times, with Neville ending up in the Hospital Wing because of the accident he got into. When on the Quidditch pitch she is lenient too, even encouraging a match to continue when dementors had flown onto the grounds. Overall she has the passion and skill but very little responsibility.
Horace Slughorn
Slughorn seems to plan all of his lessons on the fly. They are often erratic and ill conceived, with the tasks sometimes almost impossible to fulfill. He is much more concerned with who is the very best and setting up his dinner parties with students who can impress him.
Slughorn doesn't shy away from the dark magic either. When a young Tom Riddle asked him about the extreme art of Horcruxes, he didn't hesitate much before he responded with exactly what the young Voldemort wanted to hear. He was only re-hired at the school so that Harry could find out the secrets he had been hiding; otherwise he'd still very much be unemployed.
Quirinus Quirrell
Quirrell was so unexpected as the villain of the first Harry Potter adventure. He was nervous, consistently stuttered and didn't have a huge range of magical knowledge. We've since learnt that this was all an act but it was absolutely enough of a performance to convince us how useless he was.
It's a surprise that any students actually managed to learn from him as his speeches seemed to wander endlessly without much of a point and definitely without any knowledge transferred. He even acted scared of the hexes and creatures he was supposed to teach students to defend themselves against. Not only this, he then tried to kill Harry Potter in the middle of a Quidditch match!
Sybill Trelawney
Professor Trelawney possesses the ability to see into the future, creating accurate prophecies and correctly predicting events to come. Some of her art is seen as being quite flimsy and it is often made a joke out of. Even Hermione, for all her knowledge, believes that divination is merely a topic to be laughed at.
The problem is, despite how very real the magic can be, Trelawney herself isn't very convincing when it comes to teaching a class full of eager pupils. She is at best classed as eccentric and is usually speaking in riddles. She's difficult to understand and some students are actually scared by her and her apparent abilities.
Gilderoy Lockhart
Gilderoy Lockhart is an adventurer and a celebrity. He's rich and powerful. He's also a liar. He's an ill-suited choice as any kind of teacher as he can't do any proper magic himself. The only charms he is capable of are based around memory manipulation.
He's really dangerous to allow to lead a class full of students into the darker side of magic. Releasing the Cornish Pixies for instance, without any way to return them to their cage, was a big mistake and could have been more disastrous than it already was. He's too arrogant to take his eyes off of the mirror and onto what's going on in his classroom.
Dolores Umbridge
Umbridge isn't really even a proper teacher. She's hired by the Ministry of Magic to watch over Hogwarts and it's staff. She's also assigned as the Defense Against the Dark Arts professor but doesn't do a lot of practical teaching. She's hands on with punishments but not with her methods in the classroom.
She prefers that students learn everything they need to from the safety of a text book. This is completely impractical and although you'd think it would be safer, she is the real danger. In fact, it's more than likely she doesn't want to train any of her students in case they rise up against her, which of course they do.
The Carrows
The Carrows are two of the most hated professors that Hogwarts have ever known. The Death Eater twins are assigned to the school to teach muggle studies and the dark arts. They are completely vicious and vile, with no regard for the safety and well being of any of the students.
Although it seems like a little bit of a cheat to include two professors as the final item on this list, it's difficult not to mention this terrifying duo. With free reign to do whatever they see fit, they used their classrooms to torture students more than teach them anything. They are absolutely the worst teachers Hogwarts could hire and took more pride from their cruelty than from any lessons they passed along.