If you are speaking in terms of population, unquestionably Islam is the more popular religion. Over one billion people follow its tenets. It is sad that, like most missionary religions, it has been misinterpreted and at times perverted; but this does not take away from its overall popularity. Christianity would be a close second; although it has lost ground in the last century as its missionary efforts have not been that successful. It has grown more from population increase than conversion.
Where are you talking about?
In the United States, Christianity is most clearly the dominant religion. The vast majority of Americans who have any religion are Christian.
In the world as a whole, there are a couple of ways to think about this. There are more Christians than there are members of any other religion. However, you could argue that Christianity is no longer dominant because so many Christians are not devout. You have so much of Europe that is nominally Christian but where people do not really care about religion as much. If you look at it like that, you might conclude that Islam is more dominant because more Muslims are truly devout.
So it depends some on where you are talking about and how you define "dominant."