All of my friends think that Nami's past is the saddest. I'm not saying it isn't sad, I had teary eyes after reading it, but I think Zolo's/Zoro's is the saddest.
I know Nami losing her mom and being forced to join with Arlong is horrible but at least she had Nojiko. Zolo didn't really have friends.. Except for Saga who just mysteriously appeared in movie 5. Then there's Robin's, I don't know her full story because my computer broke down half way through the episode :getlost: but her past was sad with her evil guardians and the whole explosion fire thing. Once again like Nami, Robin is motherless. But I'm not really even sure if Zoro had parents to begin with, (well you know what I mean) you never really know where they are.
That's for One Piece at least, Naruto pasts are much sadder than One Piece ones. But thats a different story..