Harry Potter and the Sorcerer's Stone: Professor Quirrel (who turned out to be evil, having Voldemort stuck to the back of his head was sort of his downfall as a teacher)
Harry Potter and the Chamber of Secrets: Professor Gildroy Lockehart (very vain, turned out to be a coward and a liar and is memory-wiped by his own Obliviate spell in St. Mungo's as we speak)
Harry Potter and the Prisoner of Azkaban: Professor Remus Lupin (the best Defense against the dark arts teacher:D Was a werewolf, helped explain to Harry that his godfather Sirius was reeally good and innocent. Snape, who was angry that Sirius escaped "justice" let it "slip" that Remus was a werewolf, so ending his teaching career))
Harry Potter and the Goblet of Fire: [Was supposed to be] Alastor (Mad-Eye) Moody but it turned out to be Barty Crouch Jr. a Death Eater who had somehow captured Mad-Eye and used Polyjuice Potion to fool everyone at Hogwarts for about 9 months. He revealed himself in the end, by making the Twi-wizard Tournament cup a Portkey, sucessfully bringing about the return of Voldemort, and trying to kill Harry (again) after Voldemort failed.
Harry Potter and the Order of the Phoenix: Professor Dolores Umbridge (who turned out to be totally evil as well, secretly loyal to Voldemort, and was working undercover for him at the Ministry of Magic, and was trying to "reform" Hogwarts. Fudge [Minister of Magic at the time] was convinced that Dumbledore [Headmaster of Hogwarts at the time] was going to try and usurp his position, so he wanted Dumbledore out of power of the school. And also the fact that Dumbledore was adamant that Voldemort was back scared the daylights out of Fudge and so he lived in denial throughout this book.)
Harry Potter and the Half-Blood Prince: Professor Severus Snape (who turned bad, having secretly been working for Voldemort, and killed Dumbledore at the end of the book and ran off to join the Death Eaters.)
Harry Potter and the Deathly Hallows: the Carrow siblings, two Death Eaters that basically ran Hogwarts whenever Snape (who is headmaster of Hogwarts during the Deathly Hallows) is absent. The Ministry fell and was taken over by Voldemort, thus Hogwarts is also under his control, and Voldemort then places Snape as the Headmaster, as he trusts Snape completely. The school is at their mercy much of the time until Harry's return. But as we later learn, Snape was not bad, but actually a double-agent for Dumbledore. He used to be a Death Eater but when Voldemort started trying to hunt down Lily Potter (whom he loved although she was already happily married to James Potter), he went to Dumbledore and BEGGED him to protect them. Dumbledore did so at a price. Snape became a double agent. When Voldemort slautered the woman he loved, and her husband, Snape became truly loyal to the good cause. Bound to it not only from his word, but from hatred towards Voldemort for killing Lily. He vows to protect Harry in honor of Lily's memory. Dumbledore, at Snape's request, tells no one of Snape's vow. When Voldemort returns to power, Snape returns to his work as a double-agent, spying on the Death Eaters for Voldemort. Snape kills Dumbledore at his request, because while trying to open a Horcrux, Dumbledore sustained a curse that would evemtually kill him. Slowly and agonizingly. Snape hated to do it, but did it at his old friend Dumbledore's request, and to make Voldemort trust him completely so that Snape could continue his work as a double agent. This is revealed only in the end of this last book after Snape's death when Harry watched Snape's memories in Dumbledore's Pensieve. The memories Harry sees bring about the ending of the book. I shan't spiol it for anyone who hasn't read them:)