It is really too hard to say one had a harder life than the others.
Naruto - always alone and hated by EVERYONE, treated like a nasty plague
Itachi - had to kill his clan for peace
Sasuke - awesome big bro kills your whole family totures you then is actually good
Sasori - parents killed so you make puppet parents that just doesn't feel right
Gaara - the one person who showed him an ounce of kindness atempted to murder him and told him he is hated, treated like a weapon
Nagato - parents killed, best friend sacraficed himself in vain..
Kakashi - respected father commited suicide for being a hero and hated for it messing him up and then finding friends but loosing them and his sensei within a year/few years of each other
Kabuto - parentless until he found a foster mother, became a spy but his "mother didn't know who he was
The list of sadness is loooooong for Naruto