You guys can debate if its the saddest OP flashback ever if you want i will just point out a few things and yall can take it any which way.
For people saying we know what happened thats cool to know whats going to happen but to see it happen is something entirely different and the way Oda showed the flashbacks made all the difference.
The rumba pirates loved their captain and he loved them back. They we're pratically family. Their crew relationship reminds me of Shanks crew. The loss of a captain is huge if you saw what zoro did for luffy all of them would have done the same thing. But they were helpleess to prevent the captain from dying.
The crew gets slaughtered with out the captain everything is done for. One of the younger people on the ship looks up to brook and wants brook to train him. The promise to Laboon is finished. These we're all extremely proud men you can tell this by their motto.
Brook after death was going insane. Remembering his crew them getting whipped out didn't him the crew being there is real he feels pain so its not a dream however its just his mind and his crew mates who he was around so much are gone. The jokes the laugther songs the morning wake up routine gone.
Then the last of the crew signing so brook can record their voices and they hope that even though they die there brook can realize their dream. Their last efforts and energy were entrusted to brook to see laboon again and he keeps the tone dial in his head. Where his brain should be the only thing there is the voice of his lost comrades and the promise he must fulfill to laboon.
Amazing Oda simply amazing