Kabuto and Obito. The anime did a great job in portraying what these two characters had gone through. I didn't feel much for their sob backstory in the manga compared to the anime.
Orochimaru, at part 1 I hated him and I thought he was just a psycho who tried to destroy the village which he once lived in but now that I have seen his motives and the knowledge he has gain and also the fight against kyuubi-Naruto was kinda big thing so I started liking him.