Which character from One Piece do you think has the saddest backstory?

I think the saddest backstory is of Chopper. He ate the Hito Hito no Mi which is a zoan type devil fruit which gives him the traits of human. He accidentally ate the Devil fruit. Chopper originally had a blue nose, which caused all the other reindeers to treat him as an outcast.

He tried living in the human world, but they treated him as a monster which made him have no home. He was hungry and wandering in the lonely forest.

Then he is found by Dr. Hiluluk who treats him as a human. Dr. Hiluluk was a quack who dreamed of curing other people and being the best doctor. Chopper learns that a skull with a bone in the side means a pirate and Jolly Roger. So, when the Dr. Hiluluk gets sick from an incurable disease, he look up a medicine in the book of medicine. He sees a medicine beside the the flag. He thinks that medicine will make him fine. So, chopper goes on a harsh journey to find the mushroom. He gets beaten on the way by other reindeers, but he fights them and brings the mushroom home.

He makes the soup and gives it to Dr. Hiluluk but then he finds out that he made a poisonous mushroom soup. But, the doctor drinks the soup with pride having a epic ending. Then, Chopper goes to Dr. Kureha(A quack who is a colleague of Dr.Hiluluk)who is a professional in the field of medics. Dr. Kureha takes Chopper as a apprentice and teaches him all the medical skill she has learned. So, Chopper goes with Luffy and his crew in the aim of curing every disease in this world.