does anyone know what is the worlds most popular religion in the world?

Christianity (2.3 billion) is the most popular. Islam is second (1.6 billion).

That depends on how you define popularity.

1) Is it the fastest growing at the moment?
2) Is it the one that claims to have the most adherents?

1) At the moment, the fastest growing is probably islam.
2) At the moment, the one that claims the most adherents is probably the broad definition of christianity that includes catholics, protestants, JWs, SDAs, mormons, etc.

Yes. These people consult reputable statistical resources such as one readily finds in the most frequent hits on a standard search engine.

Statistics for the entire world can be pretty reasonably estimated, and there will be one faith that happens to be at the top of the list in terms of the sheer number of believers.

As my grandmother used to tell me, "Look it up; you'll remember it better, dear."

According to the site linked below (I have commented on some of their listings):
1) Islam
2) Christianity
3) Catholicism (should be included as Christian)
5) Agnosticism (not so much a religion, as someone unsure about their religious beliefs)
6) Buddhism
7) Atheism (not really a religion, but a lack of religious beliefs)
8) Anglicism (should be included as Christian)
9) Sikhism
10) Judaism

Money.