1. Christianity. Around 2.2B adherents. Christianity beats out Islam very slightly for the more vast hold it has on the planet. Christians are masters of deceit, they're so good that they've fooled even themselves. The Christian is a big hairy burly man who rapes one in the ass while smacking them with the Bible and yelling "Take it!". They'll shove it down one's throat, up one's ass, pretty well any conceivable place they can. When that fails they take to legislation. Despite Christians being the vast majority in America, they claim to be persecuted. If ever there were a group to want to be victims, it's Americans, it's Christians. The paramount of over-privileged, over-tolerated, over-respected groups is American Christians. Yet they whine so much about how they're marginalized.
2. Islam. About 1.57B adherents. Islam is a harmless religion. Harmless but for... Extremists who tend toward suicide attacks, genital mutilation, misogyny, the list goes on to end with plain dickishness. They share the deceit with Christians, what makes them dangerous is that they're less lazy than the average Christian. The Muslim is actually close to the stereotype. He's of middle-eastern complexion, wears a turban and blows his opponents up. Muslims and Christians share so much, but feel so apart. Muslims as a group are hated by Christians and many of them return that hate.
3. Mormonism LDS. About 14.44M adherents. The church of latter-day saints (LDS), may seem like a small movement in general, but considering it's under 200 years old, it's spreading uncomfortably quickly. What's more disturbing yet is the patent incredulity of the belief system. Basically, there's a never-ending cycle. There were once a bunch of spirits or intelligences floating around in nowhere, then [a] God(dess) comes along and offers them a chance to ascend to be like him/her. Satan however tricked some of the spirits into going with him. Mormons believe that black people are cursed for going with Satan. The ones who went with God, however, were put on earth to be tested. If we pass the test, we may become gods and goddesses of our own planets and the process repeats. This is so delusional as to be in itself harmful.
4. Chinese Folk Religions. 400-500M adherents. Varies in the degree of harm.
These may be dangerous to their constituents, but seem to be less dangerous to unwilling people.
5. Hinduism. About 837M adherents.
6. Unitarian Universalism. About 630K adherents. This is pretty much just accepting everything. It's major fault is the promotion of apathy and ignorance.
7. Wicca. About 1M adherents. That much belief in the power of will can have serious psychological effects.
8. Rastafari Movement. About 700k adherents. Bitch please.
9. Buddhism. 400-500M adherents. There are some omni-theistic tendencies, but they're less common. Buddhism to my understanding is a philosophy more than a proper religion. That gives it the sole right to the bottom of the list which should be classified "Just why?". Philosophy doesn't need another name.