What is the Holiest Mosque in the World?

The holiest mosque in the world for Muslims is the Mosque of Mecca, also known as the Great Mosque of Mecca or the Kaaba. 

It is located in Mecca, Saudi Arabia, and is Islam's most important place of worship. Muslims from all over the world make a pilgrimage, or hajj, to the Mosque of Mecca at least once in their lifetime, as it is one of the Five Pillars of Islam, a set of religious obligations that all Muslims are required to fulfil.

The Mosque of Mecca is considered the holiest site in Islam because it is believed to be where the Prophet Muhammad received the first revelation of the Quran. It is also the direction Muslims face when they pray (toward the Kaaba). 

The mosque is located around the Kaaba, a large black cube-shaped structure that is the holiest object in Islam. The Kaaba is believed to have been built by Abraham and his son Ishmael and is the centre of the annual hajj pilgrimage.