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: Professor Amycus Carrow. (A death eater who had been at the top of the astronomy tower when Snape killed Dumbledore. He was, in fact, simply the Dark Arts professor, as this was the year that Hogwarts was under Voldemort's control. His sister, Alecto, taught the modified Muggle Studies. Both Carrows had been at the top of the tower the night Snape killed Dumbledore.)