The third part of Story of Mahabharata is narrating both efforts for attaining peace without any war, and the great war leading to peace. Most of the readers consider this part as the main Mahabharata. There are heroes in the side of righteousness, and also others in the side of falsehood. But none of the character is fully white or black, each has some shade of greyness in him.
This makes this story so wonderful. Bhagavat gita is the very famous part of Bhishma Parva of Mahabharata.