Its between Kakashi, Nagato, Sasuke, Itachi
Itachi killed his whole clan, lived as a traitor alienated from his friends comrades died by the hands of a brother who hated him even though he loved him more than anything only to be edo'd and realized his planned failed. Dang
Sasuke-lived with a vow of hatred to his brother who killed his whole clan so he betratyed everything he loved to gain power. WHen he succeeded found out his brother loved him-Dang
Nagato-need I say more.LOL
Kakashi-His sensei died, best friend died when they finally developed a bond, killed rin by his own hands although he made vow to dead friend to protect her, dad committed suicide, been very involved in wars since a very young age so no childhood.
I'll let you choose.