Christianity is most common in South Sudan; with animism in second. If you want to include animism, most of the Indigenous Beliefs of Madagascar are based around animism
I think some of the more totalitarian nations have atheism as an official religion like when the Soviet Union did before its collapse and North Korea still does today.
dont play it
https://www.pewresearch.org/religion/2012/12/18/global-religious-landscape-exec/