Here it is hard to say, as both Islam and Christianity have large number of converts daily.
The more the specific religion followers the more will be the converts from this religion to Islam. Accordingly, most converts to Islam are from Christian religion. Some think that most converts to Islam are from people who had no previous convictions or beliefs. Refer to links below
A person who converts his religion is typically referred to as a convert or a convert to the new religion.
Religion
The converts were called earlier as gentiles.
diffudion and converts
There is not really a religion that is the most popular. All religions matter in life and are considered the most popular.
An Apostate Jew
A Jew that converts to another religion.
Many early converts to Islam were impressed by the good character, integrity, sincerity, good will, and enthusiasm of it's followers. Many early converts were attracted to the monotheism. And many early converts preferred any religion to death.
now Islam. Vatican declared it recently.
Christianity and the second largest Islam.
He came to the conclusion a frontal attack on the religion would not succeed . He built a school and a little hospital to gain converts