Who is our Muslim leader that we should answer when we will be asked by Angel Munkar?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Every dead person (Muslim and non-Muslim) from among the Ummah that witnessed the mission and invitation of the Prophet Muhammad will be questioned by angels in his grave about him (Muhammad).

Anas narrated that the Prophet said: “When (Allah's) Slave is put in his grave and his companions return and he even hears their footsteps, two angels come to him and make him sit and ask, 'What did you use to say about this man (i.e. Muhammad)?' The faithful Believer will say, 'I testify that he is Allah's Slave and His Messenger.’” [Al-Bukhari and Muslim]

The above-mentioned Hadeeth clearly indicates that the question is about Allah's Messenger, Muhammad .

For more benefit on the questioning in the grave and whether it is for Muslims only or for all the people, please refer to Fataawa 23100 and 258208.

Allah knows best.