Correspondant S3 points out an NRO article by John Derbyshire that claims America is the last Christian nation -- a claim made largely to contrast America with increasingly irreligious Britain. For myself, I don't think America is particularly Christian in practice, even if many consider it to be so in theory. Still, I suppose the argument could be made that it's the "most" Christian nation on earth at the moment.
Frankly, however, I don't know enough about world politics to be that certain. My understanding is that there are some fervently Christian third-world countries, such as Nigeria (?) -- although that may be more in theory than in practice as well. I've heard that South Korea has a lot of Christians, and sends out a lot of missionaries, but I don't know what effect those Christians have on their country. I simply don't know enough about foreign politics to say whether or not Christians have more influence in America than they do anywhere else.
I'm very curious to see whether or not Christianity takes hold in Iraq, now that the people's freedom of religion is finally recognized.