I have to admit that I thought the Drum arc was the worst arc in One Piece. I never really got it (sorry). To me there are two people that were real tear jerkers. Nami twice; the first being when she stabs her tatoo (to me it was the culimnation of her past story and how much she longed to break free from Arlong's ownership of her). The second being Luffy destroying her room, and thereby destroying the bond that between Arlong and Nami.
As for Robin there are too many scenes to count but the scene where Luffy is trapped in between the buildings, Robin's I want to live, and her biting the stairs are probably top three.