I would say either Gaara, Sasuke or Naruto.
Alright, the others such as Nagato, Sasori, Orochimaru... I mean they lost their parents. Sounds ****ed up in all honesty but Sasori still had his grandmother, Nagato still had his friends and group, Orochimaru was a good guy at first and had his squad.
Danzo was just *****ing because he was a coward but then again wanted people to look up to him as a real ninja. Well putting yourself in his shoes, I'm sure most of you would be scared to die but don't want people to class you as a coward. Obito cried because of his girlfriend. All that's not that big of a deal compared to these three:
Sasuke lost his entire clan, came home to a slaugherfest and thought for years that his brother was a bad guy and did it just because. Imagine your entire family getting killed and you come home to a pile of bodies and your sibling did it "just because". I'm sure you would lose yourself and see no one as your family. But then only to find out that your sibling did it for a good cause but right after you watch them die, I'm sure thats enough to send you over the edge.
Gaara, alright fair enough he has his brother and sister as his only family, but imagine your own father forces a beast inside you and then tries to have you killed. Your only purpose is being a weapon. You live in fear that your closest one wants to kill you. Your uncle who you thought was always on your side, tries to kill you. In Gaara's view at the time, he didnt know his uncle was always on his side but acted on the orders of his father.
Now Naruto, he lived his entire life without parents. Like Gaara, Imagine living in an area where everyone around you hates your guts and wants to see you dead and you don't even know why. Everyone you try to play with only for them to pick on you or their parents to take their child away from you and look at you with hatred and resentment. Then you suddenly do something good and all of a sudden people like you.
Too be honest, it's all about family. Some may have lost their parents but then they have others that care for them but instead they throw it away and use it as an excuse to go evil with the exception of Kakashi.