Ace's death.
When I watched the anime about 7 years ago, I totally fell in love him when Sengoku introduced him as Roger's son. A few episodes later, my heart was torn out of my chest when he died. I couldn't take it, I couldn't stand to think that he's dead after imagining myself as his girlfriend. It was just horrible. I started thinking and imagining all kinds of ideas of what if's just to bring peace back to my heart and mind. It ended up with a very long fanfic of how I saved Ace from Marineford and brought him to our world, and when he was healed we made a long journey by boat to go to Japan and beg Oda to get Ace to enter his world again. That was the only thing that allowed me to go on.
7 years later, when Ace remained 20 and I grew older, I finally fell in love with another, older character, King, and now I pray to Goda not to kill my new love because it would be terrible if he killed him too.
Corazon's death was sad, but there was another event even sadder, right after Ace's death: The Merry-go's. I shed rivers of tears when it was burned.