Whats the worse common religion ?

I don't like to insult people's faiths and i would expect most people to answer Islam, but my personal view is that it's Zoroastrianism because non-Zoroastrians are excluded from their family celebrations and ceremonies such as funerals and weddings, and they have, at least historically, considered it their duty not only to avoid eating "unclean" animals but actually to kill them. Having said that, i find them fascinating and i'm sure they have plenty of good traits.