Re: who has the saddest history?
Itachi and Nagato are definitely on top of the list for obvious reasons. We can probably squeeze Gaara in there too.
Sasukes past really sucked compared to Naruto, though Naruto went through his share.
I think Kakashis, while definitely not the worst, is slightly underrated. His dad, a glorified hero, commits seppuku in front of him. This messes him up mentally until Obito comes along, but soon loses his whole team which he blames himself for. Then Minato dies shortly after. (And now seeing Obito as Tobi must make the life lesson he learned seem pointless.)
As far as Obitos past... Not gonna speculate til next week at least. I still think there will be a twist to Tobi/Obito connection.