I think Elsa has the least optimism. She always pushed Anna away because she never could trust that it could be good. She didn't believe she could fix the winters in Arendelle as well.

Or Merida.
Has Elsa ever been optimistic about anything?? :/
Also, who pick Snow White?? That girl is either going to be in first place or at least in the top three.

^ Totally agreed! :D

Her first reaction when anything is happening is to panic and imagine the worst possible scenario, which is why she can't control her powers.

The poster child of pessimism.