Itachi. He was the scapegoat and killed his entire family just to keep peace in the Leaf Village, and then had to live a life being hated by the one he loved the most- his little brother. He had to live alone as a rogue ninja with the Akatsuki, who probably never really understood him, and doing horrible things for Akatsuki just to spy on them and continue protecting the Leaf Village. Itachi is a good person, so that had to have been a difficult life. And then in the end, he was killed by the person he loved the most. Now THAT is a sad life that I don't think anyone else in Naruto can top, not even Pein cause at least Pein had someone who understood him and who was there for him (Konan). Itachi had no one.
I would say Sasuke's because his brother killed his clan, yet the entire Uchiha clan was planning a coup d'etat to take over the village, and Sasuke was kept in the shadows the entire time. Yet Itachi killed his entire clan to protect the village, and his brother, so Sasuke feels sorry for himself because he killed his brother who cared for him.
Sasuke from beginning, middle, and end. Ive grown so attached to his character that cried when he fought Haku in the Hidden Mist Village..and when he left Konoha...
In my case......It's a 3-way tie between Naruto, Sasuke and, Gaara. they have all had a sad past so I couldn't pick just one.As for Itachi, he might have had one but he didn't care.