Harry Potter and the Deathly Hallows
Though he started off as a vile, er, creature when we met him at Sirius's house in Book 5, Kreacher demonstrates only the best traits of the house-elf in this book. His loyalty and undying love for his master, Regulus Black, is what brings him over to the side of good, and he becomes a helpful and fervently loyal friend to Harry.
Kreacher's story is a sad one that demonstrates the terrible mistreatment of house-elves – Voldemort's disregard for Kreacher's wellbeing means that he saw the elf merely as a tool. However, we know full well that house-elves are more than just servants; they have feelings, thoughts, and personalities of their own, not to mention powerful magic. Though Kreacher seemed like a real baddie earlier on, it was actually just his loyalty to the house of Black that made him appear that way. And, when treated with an ounce of respect, he reveals himself to be a good little guy. Kreacher leads the Hogwarts house-elves into battle against Voldemort and the Death Eaters in the Battle of Hogwarts.