Has to be Snape. He's such an interesting character, and his backstory makes me feel so sorry for him. Lonely childhood, getting bullied at school, losing Lily to a man he hated and then turning to the good side despite the fact that some people still believed he hadn't changed and didn't trust him. Lying to Voldemort while staying in his good books can't have been easy either, plus many people on the dark side hated him for staying so close to Dumbledore for so long. It seems that he was hated by most people, and no one really understood how much he had sacrificed and the risks he was taking until the end.
Voldemort is probably my second-favourite character, and I'm really glad we got to learn more about his life in the sixth book and what caused him to become the person he is.