Islam at the moment.

All three abrahamic religions.

Scientology, Islam, and Christianity.

If you can call it a religion, then I would say Scientology. Any religion that makes catholicism look sane and rational in comparison is not a good one.

Any religion that requires you to pay to play, or kill someone or something because they don't believe what you do (or don't in my case) or will kill you if you leave, sucks in my book.

Any religion with a fundamentalist sect is scary - the 3 Abrahamic religions being the worst.

there used to be a sorta cult that (if we believed their enemies description) that would send their kids to walk thru fire when they came of age, made giant mounds of feces and danced around it, and ate the babies of their enemies.

Assuming this is even half true I would pick them for scariest.

American greatness.

IslamaBAD!

All of them, but I guess Islam and Christianity are pretty much tied for worst in my mind.

Christianity...

I would say Mahayana Buddhism. Not because Buddhists themselves are particularly frightening, but the ideology of the Great Vehicle allows for no escape from the Dharmachakra or the Great Wheel. We are forced, epoch upon epoch, to flail again in our motions, with no hope of escape from the Great Wheel without the millenarian eruption of the Buddha into our daily lives. But there is no guarantee that the Buddha will sufficiently disrupt the Dharmachakra to allow us to escape from it.