Am I the only one who thought before the whole Ace and Sabo thing that Luffy had the worst of the all the backstories.
Zoro lost his friend and Rival.
Ussop lost his mother and father figure.
Nami lost her mother.
Robin lost her mother, and civilization.
Franky lost his mentor.
Brook lost his crew and captain.
Chopper lost his teacher/father figure.
Sanji lost his family, and half of Zeff's leg.
Then there was Luffy with his I want to be the Pirate King story of Shanks loses his arm saving Luffy from bandits and gives Luffy his hat. Luffy's story was lacking in the whole death part that everyone else had. Then finally at the end of the pre-time skip Luffy's at least half of it is revealed. The first thing being the death of Sabo and his quest for freedom from the prideful monarchy of wherever he was from. Then Ace's quest for freedom from the shadow of the Pirate king his father, and his desire to find a family besides just Luffy, Garp, Dadan and her mountain pirates.
When Rayleigh asked Luffy what it meant to be the Pirate King, Luffy replied being the person with the mot freedom on the entire sea. Well considering what Sabo and Ace sought in there lives that is a good meaning since it joins in with what they wanted.
Now, I do believe Luffy full backstory if yet to be revealed yet. I think Oda has something possibly involving either Garp, Dragon, Garp's wife, or Luffy's unknown mother that is yet to be revealed. But it'll probably come to light when we finally get a full meeting with these characters.