Are we talking sad or tragic. I think I should separate the two in this case. As far as the Strawhats go, I personally feel Robins is the most tragic. Although namis is pretty tragic too. But for sad, brooks was very sad and very well done, i think like others said it was best done in the anime cuz you got to hear the song.
However, and it may seem a bit outta left field, i think zoros is kinda the saddest. I mean to have this goal to beat someone and to put so much work into it and then just have it robbed from you is pretty sad, but what really puts the cherry on it is that it wasnt even caused by anyone in particular, it was just life happening. It was also just a poor innocent girl that died for literally no reason. In the case of everyone else, it was someones greed or their own delusions of justice that caused it, but to have life taken away at random, well that hits quite close to reality. There is no one to exact revenge on, no one to be angry at, no reason for it, and nothing to have been done about it, and I think for me that is what makes it the saddest.