Kakashi being the reason why both of his Comrades are dead well not obito anymore.. but at the time.
Itachi having the pain to live for ever with the guilt of killing his family and his whole clan.
Sasuke growing up with his family and all of a sudden all of that is taken away from him.
Naruto having the the Nine Tails sealed with in him growing up without any parents and everyone hating him
Gaara the Shikaku (1 tails) sealed with in him his Father and Uncle hating him and everyone else hating him for a very long time.
Obito having to be all by himself not being able to save his comrade watching rin die.
Sakura for being made fun of because of her forehead.
Lee because he could never master Ninjutsu or Genjutsu.
Tsunade her brother and boyfriend dying because she gave them a curse necklace.
Danzo for always being in the shadow of the Hiruzen (3rd Hokage) him wanting to be Hokage but never being acknowledged.
Multiple Choice if you think some people had equal amount of pains or you just feel like voting for more than 1 person
Anyone i missed out?