I do not like comparing the pain people got through; "this one had the worst" is just not a right thing to say because emotional pain is bad no matter which shape it comes in, it can not be measured(of course losing a parent is more painful than being teased becase of the forehead size; but you got my point)...
Kakashi, Naruto, Sasuke, Neji, Lee, Madara, Tobirama, Hashirama, Gaara, Kimimaro, Haku, Sasori, Konan, Yahiko, Nagato, Killer Bee, Kimimaro and others i can not remember had it bad.