What is the most expensive Diamond in the World?

The most expensive Diamond in the World today is Cullinan I, also known as the Great Star of Africa. The valuation of Cullinan I is said to be $49 million. It is set on the Sovereign's Sceptre with Cross (Crown, Cross and other ceremonial items) and belongs to the current Queen of England.

Cullinan I is the largest split of the nine pieces of the Cullinan Diamond, the largest gem-quality rough diamond ever found in history. The $2 billion worth giant diamond which weighs 3,106.75 carats or 621.35 was found in Cullinan, a small town in South Africa in 1905.
cullinan diamond
                                          The 9 split pieces of Cullinan Diamond (source: RCT)