The saddest backstory that I've seen was chopper's since it showed how someone who was abandoned by everyone found at least one person who cared for them before they died.

Oh god, Chopper's backstory made me cry, and Robin's just broke me. It's rough.

Personally, I think that Robin's was the saddest. She lost absolutely everything at the age of eight (all at once) and was made out to be a horrible criminal. Then, having no one else to turn to and being deemed a criminal anyway, she begins joining dangerous criminal organizations where she was likely made to do unspeakable things.

Everyone she ever turned to for twenty years would always betray her in the end, so she stopped trusting people altogether and began betraying them before they could betray her. Then, she finally met the Strawhats and made true friends. Friends so true that after a lifetime of betrayal, deceit and just trying to SURVIVE, she was willing to throw away her own life in order to spare theirs.

I found Brooks backstory the saddest. He had a very loving crew until they were attacked and the all slowly starting dying singing "Binks Sake" Then he died and was brought back to life to sail the gloomy sea for 50 years. It makes me lonely just thinking about it.