My parents are separated. And I live ith my father. My neighbour whenever possible tries to tell me something bad about my parents and that my father is a bad person and that I am bad too. Also says that my parents have a bad character. I just can't hear that anymore and also know that my parents are not bad and want to give him a good answer so that he does not interfer in my personal life. He keeps on tpunting me about my parent's unsuccessful marriage.

People like that are just so shallow and or insecure.
They will put others down and then often the same thing happens to them.

I really don't know what you can say because you could come up with the truest wisest reply in the world and he will only think you are smart mouthing him because it came from you and you are a kid.

There is a saying for people like that that goes 'consider the source'.

Have you told your mom he is doing this and it is annoying you?