Well the top one is easily Gaara. Not only did he have to expirience the lonliness of being a Jinjuriki, his own father tried to have him killed. One's family is basically the core heart and center of developement during childhood...needless to say look how Gaara went up! He didn't ever get to know that the his mother and Uncle (last guy to try and assasinate him) truly did care for him until he fought his dad.
after that it depends on whether you find it sadder to grow up alone or to grow up with people, but for them to despise you.
In my personal opinion, I find it sadder to grow up having a family, but they think of you as trash which is why I consider Hinata to have one of the saddest childhoods. Here she is supposed to be the next Heiress to her clan, but she lacks natural talent....she showed that she easily had the skill to train hard and would have ended up an amazing Kunoichi, but that didn't happen. Instead, Hiashi decided that she was trash because she lacked natural talent. Because of this it caused Hinata to lose faith in herself and basically be depressed. On top of that, her cousin absolutely despised her and even found her spoiled (which she wasn't) simply because she was a main branch member (apparantly he never noticed how the main branch seemed to view her). The fact that Hinata has her sister, a cousin, and even a father (maybee a mother), and yet they all basically put her down and turned away from her is basically as bad as something like Naruto who grew up alone.
I could imagine how sad it is to grow up alone, but wouldn't it be so much worse to be in just about the same situation....except unlike the person who is alone by force, you are alone because the ones that should love you and ecourage you simply reject you?!
Now if by no means was Hinata's life HARDER than those like Naruto or Sasuke....easily just as sad in my opinion.
If you, however, find lonelyness sadder, then Sasuke or Naruto
Naruto had to deal with growing up alone for much longer, but Sasuke still had to go through growing up alone. The difference being that Sasuke had the horror of watching his family and clan be murdered before his eyes. We all understand Naruto's pain.
SUMMARY:
If one considers Family Hating One sadder
1. Gaara
2. Hinata
If one considers lonlyness sadder
1. Gaara
2. Sasuke/Naruto