I would have to say those that worship the Devil. The Moslems and the Jews are trying to worship God, but they are in error. The same could be said of other religions. However, what could be more evil than those that not only reject God, but choose to worship Satan instead?
I refuse to name a "more evil" or "most evil" religion, but I did want to add my two cents on this post. I have studied Islam and I've had Muslim and Jewish friends throughout my life. Now, I am not 'seduced' by any aspects of their faith and I still wholeheartedly believe that Christianity is the true faith, but they worship one God and I have had many amazing discussions about God with them and I believe that they are sincere in their pursuit of God. Mistaken and in the wrong faith, yes, but anyway, that's not the point. Wouldn't someone who rejects the existence of God present more of a problem?
And I love the Arabic phrase "enshallah," (as God wills it, if it's God's will, there are a ton of ways to translate the phrase for colloquial English) and I wish we used it the way that some Arabs do, when in describing any future action. Because really, we don't know if we have a tomorrow unless God wills it!
Of course there are evil people professing to practice Judaism and Islam (and Christianity for that matter), but I have a hard time naming them the most "evil" of the faiths. Actually, I have a hard time contemplating that there's a scale of evil, more evil, and less evil. Does God have a chart, rating them from 1 to 10 on the most evil list?
"Well, Mormonism is only a 3 and he DID call Jesus Lord and Savior and his heart is there...so I guess he can get into Heaven. Set him straight before he comes in though, okay?"