What is most exspensive diamond in the queens crown?

Since none of the current crown jewel components are available for sale, or valued otherwise, it's not possible to give a monetary-amount answer to your question.

However, the largest diamond in the Imperial State Crown -- one of many, as below -- is the Cullinan II, or Lesser Star of Africa Diamond.

The Cullinan II -- again, one of many -- weighs 317.4 carats (63.5 g), is rated as D or E colour, Flawless, and and is the fourth largest polished diamond in the world, according to Wikipedia, also below.

(Wikipedia lists an estimate for all of the Cullinan diamonds -- nine in all -- cut from the massive 3,000+ carat raw stone as US$400 million.)