Followed closely by Islamists. As someone else has pointed out, Islam is a serial killer of entire cultures and peoples. What it does not kill, it enslaves and impoverishes. Communists and Islamists share a murderous history and an intolerant and uncompromisng attitude towards those who don't believe in their nasty little utopias.
I really consider islamists to be in the same category as socialists.
Their so-called religion is merely a political system masquerading as a religion. The whole idea of a caliphate is to obtain and expand the power base by either conversion or death. And like the Mao, Stalin and Hitler they were interested in world domination, but the bastions of freedom stood against them.