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 Gilderoy Lockhart (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. She was an ally to Voldemort and was working undercover for him at the Ministry of Magic, and was trying to "reform" Hogwarts).
Harry Potter and the Half-Blood Prince: Professor Severus Snape (Became headmaster during Harry Potter and the Deathly Hallows).
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),