koso こそ

Particle stressing something that is worth mentioning.

It essentially takes the place of the particle ga or wa after the subject.

1. After noun
これこそ私の欲しい本です。
Kore koso watashi no hoshii hon desu.
This is the very book that I want.
私こそお礼お言わなければなりません。
Watashi koso orei o iwanakereba narimasen.
I(not you) should expressing thanks.