damn grey knights...they are cool, but they have essentially become SPESS MEHREENS WITH MORE SPESS MEHREENS!
the space marines are elite, brutally trained, rare, genetically modified super humans.
grey knights are space marines that are EVEN MORE ELITE, even MORE brutally trained, EVEN MORE rare, EVEN MORE genetically modified, SUPER SUPER HUMANS!

plus kaldor draigo and matt ward have screwed everything over--but im convinced hes in a coma, and he will only wake up after he solo's all four chaos gods in a battle, and beats them all at once. when he wakes up he will be getting a slaaneshi treatment, getting some just as planned from tzeentch, have his limbs being torn and re-torn off etternally, and have every disease that ever existed and ever will exist.

Still better than Centurions....

And how you dare mentioning Draigo...... That guy is just.... Like the Calgar of Grey Knights!

EDIT: Most crappy thing is anyway with them is that it was said the Grey Knights cannot be corrupted because of the Emperor's Geneseed.... SO HOW THE HECK DID THE PRIMARCHS GO HERETIC?!

centurions are cute. grey knights are such a big part of the lore that i cant dismiss them as cute. and good question...

Well that happens if you do not know your lore as a lore writer, lol!

Yeaaah explain in M41 that all grey knights are uncorruptable because they are the sons of the God-Emperor himself, but the Primarchs which were made out of his seed were not immune.... yeeeeaaaah...... Why-ever the closer genetical children should be less immune than those distinct genetical enhanced Warriors ^^

I'm guessing it's because the Primarch's DNA was changed for each one, which resulted in their mutations and quirks. The Grey Knights could have an unaltered gene-seed from the Emperor, resulting in a higher resistance to the corruption of Chaos that the distinctly altered Primarchs and their legions did not possess.

Mhhhh that could actually be, but would be kinda plain stupid to not give the primarchs those parts with uncorruptable-ness ^^

As the Horus Heresy showed, the Emperor was not perfect, and he made mistakes. Plus, the Primarchs were taken from him before they were ready; perhaps he had something planned that he couldn't do or didn't bother with when he found them?

In my opinion, the Primarchs were corruptible because they weren't raised by the Emperor from birth. They all spent the first parts of their growing lives on separate planets to their own devices and influences. The Grey Knights are raised, trained, and psycho-conditioned (even have their memories wiped) from the start of initiation on up. They were raised and trained in a controlled environment. The Primarchs weren't until later after the Emperor found them again, when they were all grown up into adulthood.

It's not who had the most direct gene-seed from the Emperor, it's how they were "raised."

Indeed, we know how the Night Haunter was brought up, and he turned out terrifying xD

honestly, i think "incorruptible" is a stupid idea. Nobody is incorruptible--everybody has a 'price'. However, depending on circumstances, you could refuse what would normally be your 'price'. You could also be tricked, and end up working for the enemy, only to realize too late after you have daemonic wings. justasplanned.
inquisitors have been corrupted--more iron willed people than the grey knights themselves. there are two possibilities. you either die in battle, a loyalist fighting for humanity, or you live long enough to see yourself become the very thing you hated most.