My first choice is Gramma Tala, but her death is really what gets the movie going. So I'd gotta say Bing Bong. It would have kept in a lot of tears from younger me.
Tadashi. The old ladies were at the end of their road and i don't really care about the others, but Tadashi was killed in a fire he ran bravely into to save someone, he didn't deserve to die.