My saddest was my first sim death, a young mother.
She took her daughter to visit my brother's mermaid family at the beach, to work on her social aspiration. While the girl was chatting with the mermaids, the mother made the foolish decision to go swimming. I didn't even know she was swimming, until a notice flashed onto my screen that my sim was dying from drowning. I immediately hit pause to see what was going on. She apparently went swimming while tired, and fell asleep in the water. I sent her husband and daughter out to plead for her life, but the Reaper just swept in and took her without giving them a chance.
My brother decided to save as a separate game, and see if she was still alive in the previous save (he had autosave turned on). She was. Then I had to decide if I should just go back to the last save, before she died, or continue playing her dead. I decided to try playing out the tragic loss (but kept the old save with her alive just in case).
She was very in love with her husband, but they had recently been fighting because he thought she was being too flirtatious with other men. In fact, he almost died from anger. (I didn't know at that time that you could.) So she was feeling sadness that day she died. So I roleplayed her husband having guilt about his jealousy, and wishing she was still with them.
For the daughter, it was especially tragic because she was with her mom when she died, and blamed herself for her death. She didn't do anything wrong, but she felt that if her mom didn't bring her to Sulani, she'd still be alive.
It was sad story, but I kinda enjoyed the tragedy. The father searched everywhere for her body (urn), but couldn't find it. Her husband and daughter grieved for several days over her loss. It was so sad, that it was actually kinda beautiful. It was like I was writing a tragic novel.
Then one day I found her urn in the household inventory. She died in Sulani. How did her urn get in the household inventory? Well, that was cool. Now they would have a place to mourn her.
Her husband was so grieved, that he built a garden sanctuary on the roof to honor her, and set the urn there. They were always going up there to mourn her, all the time. It was so sad.
Then her ghost came back to them. They were so happy to see her.
Her husband apologized for everything he said, professed his love, and asked her to join their household. She said no, but she kept coming to visit them.
Then one day her daughter asked her to join them, and she said yes. So they were a happy family again, except the mom was a ghost.
The daughter seemed to be confused, because she kept going up to the rooftop garden to mourn her mom, even though she was living with them. Apparently having a ghost for a mom just wasn't the same, and she continued to mourn her death. Eventually the father decided to hide the mother's urn in the household inventory, so the little girl would stop mourning her mom right after talking to her. Then the girl was finally able to get over it, and come to be happy that at least her mom was still with them in spirit.