Eritrea probably has the largest percentage of Christians of the countries on the list (I don’t know the figure), followed by Syria which is about 10% Christian.
I just checked the Wikipedia article on Eritrea. It cites a US Department of State estimate that 50% of the population is Christian, and a Pew Research Center estimate that the population is 63% Christian (part Eritrean Orthodox, in union with the Coptic Church, part Catholic, part Lutheran). The government does not allow unrecognized churches to worship freely such as Jehovah’s Witnesses or Seventh-Day Adventists. Most of the rest of the population is Muslim with 2% other.