User blog comment:Neowitch/Oh, cruel fate.../@comment-972068-20111104205606

Luffy got a promise, Zoro got a dead friend, Nami got 10 years of misery, Usopp got a mother dead but loads of entertainment the rest of his childhood, Sanji got a bond with a new "father", Chopper went through pains of misery, Robin went through worse pain, Franky was hated but happy, Brook died happy knowing his promise to Laboon will come true.

Zoro, Nami, Chopper, and Robin definitely had the worst pasts. I vote Robin.