Nico Robin easily had the saddest back story. She was treated with hate by her fellow children, and when she was just like 9 years old, she had the navy obliterate everyone and everything she ever held dear to her. She was then sent on the run with nowhere to go, always haveing to watch her back, for many years.
Brook comes closest after that. He lost his crew to a poison, and couldnt fulfill his promise to laboon. He had to endure many many years of loneliness. But aside from just being alone, he did not suffer. When he died with his crew, they died happily. (unlike robins people)
So honestly which do you think is sadder?
A young little girl, haveing her village of harmless people, and her island and mom completely obliterated, and then being forced to suffer for 20 years while running around, never being able to trust anyone?
or
Brooks crew getting poisoned, and dieing, leaving him all alone for many years.
Brook at least had happy memories to look back on. Robin didnt really have much of that. Brook was greiving while Robin was scarred for life.
Now it will be much happier when brook sees laboon than when Robin finds out the true history.