Nesh and Tzeench are both vastly overstated as a threat. Their 'corruption' tends to affect the upper 1%, which, as countless examples in canon have shone, are easily replaced by another beaurocrat once the corruption has been located. Both are easy to weed out and fairly conspicous for so-called "subtle" deities.

The upper 1% tend to drag entire planets into the mess, though, and replacing them tends to come at a rather high cost.

Nurgle on the other hand can only really rely on the lower end populace to establish starting influence, but also has a much lower general appeal. People are usually more interested in art, sex and fine foods than destructive nihilism, and ultimately, a bunch of plague bearers hiding in the sewers just do not seem as threatening to me as a corrupted Ministorum Cardinal or Planetary Governor with control over professional armed forces and the ability to pass and enforce new laws.

It's a lot easier to fight Chaos when you can clearly see it. The real threat is what you cannot see.

And from what I've read, all the Chaos gods have some rather potent champions capable of wreaking havoc.

Nurgle doesn't gain influence through manipulation or seduction; that's the realm of Tzeentch and Slaanesh (heck, even Khorne, to an extent with his "follow me to be the best warrior"). Nurgle, on the other hand, says "Oh, you've got a horrible wasting disease, let me help you with that" failing to mention that he's the one who both invented and gave you the disease in the first place. People might want to convert to the worship of the other three, but they feel compelled to turn to Nurgles service; it's that or a slow, horrible death.

The Slaaneshi or Tzeentchian Cult in the upper echelons of society is pretty visible if you look in the right places. Nurgles influence spreads across the masses and nobility with indifference; are you (as an Inquisitorial agent) going to search every hovel and basement for signs of the infected when you don't even know there's even a Cult to be found? "Doing the rounds" of a few parties and civilised households of the nobility is easy. Searching even a single planet, with its population of billions, is a whole different prospect.

You can resist the lure of Tzeentch or Slaanesh with willpower alone, not so Nurgle; his corruption is much more physical in nature. Grandfather Nurgle accepts all into his bosom with equal love, distributing his blessings just as magnanimously. If you descend into the bowels of his sanctums (assuming you can find it in the first place), you expose yourself to him in both body and soul. You might go in a devoted servant of the Emperor, but when you return you may already be on the way to being one of Nurgles children without even knowing it; a carrier of one of his myriad plagues. As your body decays, through no choice of your own, you have two options; to accept Father Nurgles blessing and fall to the servitude of Chaos, or to end your life, damning your soul to the Chaos God of Death and Entropy. Only the purifying fires of the Ecclesiarchy might be able to save you, if you truly hold the belief of the Emperor in your heart, but you had better be sure of yourself if you make that choice, for you'll get no second chance from those fanatics...

Nurgle still can't just snap his fingers and infect everyone, though - it takes a catalyst (else the whole Imperium would have been over long ago), and the very reason that people usually don't just start to worship Nurgle like they do with the other gods is what I'd consider a weakness as it makes expansion almost entirely dependent on activities of existing worshipers or Warp-related incidents generating corruption.

Consider too that Imperial nobility is fairly decadent by default, which can make it difficult for controlling organs to discern between normal parties and cultist activity, not to mention that the Inquisition isn't exactly numerous (although this depends on where you look -- Dark Heresy differs from GW source material in that cells of independently operating Acolytes aren't part of the original description, but rather that everything is done by the Inquisitor him- or herself with the Acolytes merely part of their immediate retinue).

Personally, I'd consider a group of security enforcers or PDF troopers shotgunning their way through aforementioned hovels and basements more likely than a specialist purity control squad showing up in a noble court ... although in both cases it strikes me as looking for a needle in the haystack, given the numbers involved.

I would go with Nurgle as the most dangerous. Death and more importantly Decay are his domain. Not only on a personal level, but as concepts. That is the reason why the Ascellon sector is of so much of interest to him (stated somewhere in the book, also expressed in the fact that the daemons in the Corerulebook are all his). But it's not only the Ascellon sectoer: The whole Imperium is in a constant state of Decay, slowly sliding towards the inevitable collaps.

So while Khorne thrives on the wars constantly fought in the Imperium, Tzeentch strives on the schemes inside the different Adeptus, Slanesh strives on the decadent and perverted noble, Nurgle strives on every aspect of that, because they are all expressions of the same fact: The imperium is doomed to slow and crawling death.

Once again. Tzeentch doesn't thrive on the schemes inside the different Adeptus, it's just his prefered way to do things.

He strives on every man who don't like current state of his life and want it to be changed. Any way. Any kind of will to change or ambition feeds Tzeentch. Anybody who opposes sliding towards the inevitable collaps feeds Tzeentch.

That's why Nurgle and Tzeentch are bitter rivals. They have quite the opposite domains.

I think madMAEXX has a point; the powerful and mighty are far more likely to scheme, though. Most common men and women simply accept their fate, and if they scheme at all it's something rather mundane such as "how can I make sure that other baker doesn't ruin my business". As such, while Tzeentch truly does profit from the hopes and dreams of most people, the real treat are those who have massive ambitions and the means to realise them, and that usually means nobility.

It's similar to how you could say Slaanesh or Khorne, too, benefit from every single citizen, because everyone is bound to feel bliss or anger at some point, but that's a far cry from those who overdo it and truly attract the gods' attention.

In the end, of course it's all a matter of interpretation. 40k cosmology does allow for a variety of views, after all.

Yes, of course - that's why Tzeentch IS the Great Schemer. But even great warlord who go and kill everybody who stands between him and throne feeds Tzeentch. He feeds Khorne too, of course, and maybe Slaanesh. But never Nurgle.

That warlord doesn't feed Tzeentch only if he kills just for killing or Khorne, not for any kind of personal ambition.

Yes, of course - that's why Tzeentch IS the Great Schemer. But even great warlord who go and kill everybody who stands between him and throne feeds Tzeentch. He feeds Khorne too, of course, and maybe Slaanesh. But never Nurgle.

That warlord doesn't feed Tzeentch only if he kills just for killing or Khorne, not for any kind of personal ambition.

A Warlord cuts a bloody swathe across a solar system. Tzeentch gave him the idea to change the status quo. Khorne fuels the fire of his bloodlust. Slaanesh gives him pleasure at his conquest. Nurgle reaps the reward of every corpse he leaves in his wake, bloated and rotting, spreading disease among what little population remains. He revels in the inevitable deaths of those not party to the slaughter as they suffer and die because their planetary infrastructure cannot support itself any longer.

In the short period of the Warlords conquest Tzeentch, Khorne and Slaanesh get a short-lived morsel of power. In the years to come, Nurgle feasts on the ashes of what remains. Why do you think he's most often depicted as grotesquely obese?

I could make similar arguments for the primacy of any of the Chaos Gods, but I only wish to demonstrate that Nurgle has more influence than you appear to be crediting him.

Tzeentch gave him the idea to change the status quo. Khorne fuels the fire of his bloodlust. Slaanesh gives him pleasure at his conquest. Nurgle reaps the reward of every corpse he leaves in his wake, bloated and rotting, spreading disease among what little population remains.

Yup.

And you can notice: " ...and yeah, for killing I vote for Nurgle. He is apathy and stagnation and decay, something described as main Imperial problems." © me.

So yea, that corpses will feed Nurgle, but not warlord himself. Well, not even corpses as they are - as I understand Warp corpses are just items from Materium; but agony, feeling of doom, apathy of dying, hopelessness, humility before destiny.

I vote for Nurgle not because he is the powerest. No. Just because that, if I get Warp right, it will lower the pressure of his emotional spectre on humans, with Tzeentch spectre (ambition, wish for change, hope) will arise. Not for forever, no - until new Lord of Decay appears, and he will, sooner or later.

Yes, that ambition can ruin the Imperium. But, to be honest, I believe current problem of Imperium is LACK of ambition.

Awesome. Interesting points. Many thanks for the contributing, all.

I'd say that Tzeentch is likely the most dangerous Chaos God purely for the fact that he's such a tricky bastard that he's likely planned for any eventuality, including his own possible death, so he's always be able to **** you over no matter what you do. Slaanesh might be more dangerous in the long run though, with the Eldar Craftworlds slowly winking out, the influx of Eldar souls will only make his power wax as time goes on