It's definitely Sasuke. I've come to find the character extremely annoying, but there's no denying that he has it the worst. His entire family massacred in front of him at the age of 7, by his older brother, who he thought loved him so dearly. With Gaara and Naruto, they never had the love from the start and sought it. With Sasuke, he had everything, and then everything he knew was just turned upside down. The trauma at that age... and getting Tsukuyomi'd as well - LOL, that's just the icing on the cake in terms of how horrible it all was.
Gaara is closely the second; he has the trauma of not only being hated by everyone, but people actually trying to kill him, and not being able to get any proper sleep because he's the Shukaku's host. How can a child who has grown up with that from birth EVER process such a thing?
Naruto is the only one that hasn't really gotten anything to complain about. All he had was the hatred, the loneliness and the lack of family. But even then, he had people like the 3rd Hokage and Iruka looking out for him. It's not like anyone he loved dearly tried to kill him or his loved ones lol.