Tough question. Ok, for me Sanji, Zorro and Usopp were the least sad in that order because Sanji didn't lose anyone he cared about and had a good life afterwards. Zorro lost someone but you couldn't really get emotionally invested in his story and Usopp lost him mum and his dad left but he had a good life otherwise and both Zorro and Usopp's loved ones died of natural causes, they weren't murdered or helped along by either of them like Chopper and Franky. In terms of quality of life Robin, Brook, Nami and Chopper had it pretty bad. Robin was badly treated by her foster family and the other citizens before the buster call and then had to go on the run for 20 years after. Brook had to live in the dark alone with the bodies of his other crewmen for 50 years. Nami had a decent life before Arlong even though she was poor but she was forced to slave away and steal for the guy who murdered her mother for 8 years. Chopper was rejected by his fellow reindeer and was then shot at and chased by humans. I haven't mentioned Franky and Luffy because Franky could've been abused by his family since they dumped him in the ocean but we don't see that and if you think about it Luffy grew up in pretty bad conditions but he was surrounded by people who cared about him so he was happy mostly. In terms of body count, on a personal level Robin lost her mum, all the archaeologists who were kind to her, her giant friend and her foster family, though she might not have mourned them as much. Impersonally, everyone from where she grew up was killed. Luffy lost Sabo and a whole lot of people were killed in the fire in the dump. Also I think the fire was on par with what happened to Robin, not in terms of the number of people who died but because of how senseless it was to kill all those people just to make the kingdom prettier for the celestial dragons. Brook lost everyone in his crew. Nami lost her biological parents and Bellemere and people in the town killed by Arlong. Chopper and Franky both lost their mentors and I think some other people may have died in their back stories but I don't remember. In terms of long term trauma, all of them have an emotional scar of some kind because of their past that affects their present behaviour. Sanji won't let anyone go hungry, Robin let the people she hid amongst die, Nami is obsessed with money and used to hate pirates, Luffy wants to be strong so he can protect the people around him, Franky wanted to give up his dream to be shipwright on the best ship in the world out of guilt etc. However, most of them get on with things and have generally cheerful personalities and hardly show their trauma. Nami and Chopper acted the most traumatised, especially when Nami stabbed herself. In terms of guilt, Franky was the most guilt ridded because his ships were stolen, used to attack his home and got Tom killed but Chopper also gave his mentor a poisonous mushroom thinking it would help him and he must feel guilty for that even though his story focuses more on his mistrust of people. Luffy also has cause to feel guilty because he was tricked into helping set up the fire at the dump but he hasn't shown any guilt for that and he might not have made the connection. Also the people from the dump weren't personally connected to him though it'd still be natural to feel guilty for inadvertently getting them killed. I think Robin has the saddest overall backstory but the others' are still really awful too. I actually cried more when Brook's crew sang for Laboon as they died than when Robin mum was killed.
Edit: I didn't know my post was this long til I posted it LOL!
