Assalamu alaikum,
Could please tell give an answer to the following please.
If someone calls a another a person, a munafiq because he lies and sins. And in return, the person replies by calling him a kaffir, who is to blame and which word is the most sinning, a Munafiq or a Kaffir?
Jazakamullah kahirun.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Undoubtedly, both parties are guilty, for Sharee‘ah forbade accusing a Muslim of disbelief or hypocrisy with no evidence, as we have previously clarified in Fataawa 84943 and 91677.

Also, it is impermissible to ascribe hypocrisy to a Muslim only because he has one of the characteristics of hypocrisy. This was previously clarified in Fatwa 86428.

In principle, hypocrisy (of belief) is more serious and more dangerous than mere disbelief, which means that the one who accuses another of it might be more sinful. But, at the same time, if the accuser intends only hypocrisy of action on the basis that the person whom he accuses tells lies, then, surely, hypocrisy of action does not take a person out of the fold of Islam.

Allaah Knows best.