Bluestar.
Why?
Let's take a look at how quickly her life disintegrated. She became insane, senile; her judgment was clouded, impaired by the constant fear of betrayal and deceit thanks to Tigerstar's actions. Her descent into madness was what forced Fireheart into his role of being a leader- and she's not the only leader to have done this. Her death was one of the happiest moments in the books; not because she was a terrible character, but because her suffering and paranoia were ended. She managed to die peacefully because she knew her son and daughter had forgiven her; she knew that ThunderClan was in good paws; her death was merciful, compared to the idea of her living for much longer.