Sasuke, closely followed by Nagato and the Ame orphans. I have mentioned real life cases of people driven mad, much older than Sasuke, by witnessing a single murder of their parent at the hands of strangers. We are talking about an entire clan and a scene of littered corpses and human shambles, and the murder of his own parents, nothing but an indiscriminate slaughter, and that too at the hand of your own brother.
The trauma is unthinkable. And to relive that, being tortured by your own brother? Sasuke's trauma is too profound to even imagine. Nagato - and the Ame orphans - was the same. He saw his parents death at the hand of Konoha soldiers and then his friends death, again because of medling of Konoha, knew starvation and misery of war. He sought to change it, only to stumble down hard. Other than that, Gaara is somewhat of a contender. Kabuto's pain was more profound. He found shelter and had to kill the woman whom he cherished as his own mother because of Root's scheming.
Gaara's pain was only legitimate till the moment he didn't grow up. After that, he had a choice to leave his village. He didn't. Self-inflicted misery. So, here's my list.
1 - Sasuke
2 - Nagato and the Ame Orphans
3 - Kabuto
4 - Gaara till the point he didn't grow up.
The rest are whiners.
P.s: Naruto and Killerbee's pains are worse than Sasuke? You people are kidding me, right?
Easily garra.
Sasuke coming home to a dead family is not as bad as garra coming home to his father's hitmen. At least sasuke knew love.
Coming home to a bloody street and corpses littered every where and seeing your own brother slice your parents, and then reliving it all in Genjutsu is
not that bad? Hilarious.