The correct answer is Communism. And this is a matter of fact.
But the chances are you may think that you disagree, or regard them as equally bad, or that it is too close to call.
However, if so, you are mistaken, and for one of two reasons:
1. Most likely, almost certainly, you do not know enough about Communism. Even I, who am no friend to Communism, continue to be surprised by what I did not know about the evils of the USSR. It has only been during the past year I have begun to appreciate this, and even in the past week some major new horrors have come to my attention. But don't take my word for it, find out for yourself.
2. The Texas Sharpshooter fallacy
I described the TSF here: http://charltonteaching.blogspot.com/2010/06/measuring-human-capability-moonshot.html
The way it work in this instance is that Nazism is defined as the ultimate evil - then other evils are measured according to how closely they resemble Nazism. Naturally, when this is done to Communism, it seems less evil than Nazism.
The relationship between ideologies (over the past couple of thousand years in the West) is as follows:
1. First came Christianity: primary sin = pride; primary virtue = love (i.e. the type of love which is agape/ charity). These defined ultimately in terms of spirituality, transcendentals, other worldly factors.
2. With leftist/ progressive atheism (e.g. Communism) the primary sin became selfishness; the primary virtue = unselfishness (a.k.a. altruism). These being defined in this worldly and materialistic terms - as 'worldly goods' ('goods' including all valued materials factors such as money and also socially-defined factors such as status).
Unselfishness is operationalized as altruism on behalf of others - e.g. other classes, other races, other sex, animals, climate, the planet...
3. Rightist/ reactionary atheism (of which Nazism is a type) reacts against the self-hatred and suicidal effects of leftist altruism on behalf of others, by reversing the morality of unselfishness to regard this-worldly materialist selfishness (under some communitarian description) as a virtue rather than the primary sin.
(In this sense, Nietzsche was indeed the philosopher of Nazism.)
Selfishness is operationalized by right-wing atheism as distributing worldly goods to one's own class, nation, empire, race, sex or whatever.
To be paradoxical about it, Nazism is aggressive altruism on behalf of oneself!
Both Communism and Nazism are relativistic/ nihilistic - they do not aim at a specific state of affairs, but a permanent revolution in a particular direction - secular leftists aim at continually increasing altruism to others, secular rightists aim at continually increasing selfishness.
Hence atheist ideologies of both right and left are capable of unrestrained evil, so their regimes are the worst in human history - but atheist leftism is capable of attracting vastly more widespread and sustained support and idealistic zeal by its pseudo-morality of un-selfishness.
Hence Communism has spread almost everywhere and accomplished (and is accomplishing) vastly more evil than Nazism - which was a narrow and unsustainable product of unique circumstances.
So - Christianity promotes transcendental love, Communism promotes worldly unselfishness on behalf of others, Fascism promotes worldly selfishness.
Leftists and progressives therefore regard Communism as intrinsically superior to Nazism - in a way that takes no account of evidence, since they see Commuinism as having the highest possible human aspirations - albeit they are usually corrupted.
Leftists regard Nazism (and other forms of secular rightism) as intrinsically evil because its advocates openly promote their own interests: its primary morality is selfishness. Since this is the exact opposite of leftism - indeed, an exact inversion of leftist morality - it is the ultimate evil.
(Note: Leftists also regard supernaturalist Christianity as intrinsically evil because it promotes non-worldly goods, which do not exist; thereby ignoring or neglecting the moral centrality of enforcing the altruistic distribution of worldly goods.
But, for leftists, Christianity is not the ultimate evil, since it is not the exact opposite of leftism. Rather, orthodox Christianity is seen as a hypocritical mask for secular rightism - which is seen as primary. Christians are therefore seen as promoters of selfishness who cleverly disguise it under a cover of nonsensical transcendental aspirations.
Explicit, open, un-ashamed secular rightism is the primary enemy.
So, Communists fear Nazis - because they understand and respect them, but despise Christians - who are seen as fools and cowards.
Communists want to fight real Nazis (if they think they can win), but want to exterminate Christians (as mere vermin.)
So, for leftists, the difference between the mainstream secular right and Nazis is merely that Nazism is more honest and brave: the secular right with the gloves-off. Mainstream rightists are seens as nothing more-than - or other-than - feeble Nazis.)