As I run most of the games I’m involved in I may have a different outlook about my favorite books than a player might. First of all let me just say that it impresses me how I have never been disappointed with any of the Dark Heresy books that I’ve purchased. Hats off to everyone at Fantasy Flight that help turn out one of the best role-playing games I’ve ever encountered!
As I said before I love all the books that have been published and though I believe that Creatures Anathema is one of, if not the best “Monster Book” of any RPG I’ve ever run, my choice for best sourcebook would have to be Disciples of the Dark Gods. I find this work to be an almost endless source of plot devices and conspiracies for my games. I never grow tired of reading through it and at anytime I feel the old “GM block” sinking in during prep time all I have to do is start thumbing through DotDG and the ideas start flowing like wine!

So…What’s your favorite sourcebook?

I really love them all but my favorite one past the main rulebook is the Inquisitor's Handbook. You get so much in that book. New ways to use old skills, new gear, alternate classes, crafting rules, and more. It gives you tons of more options for playing the game. Plus, it has a good amount of fluff and plot seeds as well. Overall I love the thing and I would strongly suggest that anyone who already doesn't have it should pick it up.

The Creature's Anathema is great, but the Inquisitor's Handbook has been the most useful for me so far.

BYE

Inquisitor's Handbook. No doubt at all.

Granted I expect Ascention to be at least this awesome too. I have high expectations. So far FFG has lived up to them.

Radical's Handbook. Really helps me to make a BBEG that really is out of my mindset (since I'm a Puritan, through and through). Other than that, has some interesting options for characters, some cool gear, and some happy fluff to read about.

I'd have to say DotDG first and foremost. If i ever need an idea for a big over-arching plot, then it will give me that idea, no problem. RH is a close second in this regard and for the wealth of info it gives on the =][=, more conspiracies, and, again, story ideas even though i have yet to use it. On the heals of RH is CA. CA is chock full of so many plot seeds, it's great for the smaller scoped stories or a chapter or two in a much larger one. Just flipping through it brings a cascade of "what if this were used by X? but then that came in.." kinds of thoughts which is really helpful. I only rate RH higher because, while it helps germinate ideas but not quite as many as CA, it has a hell of a lot more info on the Inquisition and the political temperature of the Calixis sector and a wider range of ideas that appeal to a bigger story much like DotDG puts forth.

So, ya, those are my favorites. I couldn't give two shakes about expanded or new rules (the less rules the better IMHO), or stat blocks as it doesn't take any time to come up with a number --coming up with a description or idea for something that is unique, bizarre, and expands understanding of the world, that's pure gold and what i look for in a supplement. I like setting information and evocative ideas that really gets the old brain juices flowing so i can whip up a good plot for my players and the three books I listed do that quite well.

Disciples of the Dark Gods without doubt, closely followed by the Radicals Handbook

DotDG has sooo many cool details, storys, plothooks,... And it's fun to read!

DotDG gets my vote for being such a great sourcebook for details, mysteries, adventurehooks and even whole storylines. Inquisitors Handbook is okayish but simply too much of it is pure guns & ammo show for minmaxing combatgamers.

Disciples of the Dark Gods for me I think, closely followed by Creatures Anethema, probably for the same reasons as many of the rest of you. DotDG has so many ideas that can turn into really nice adventure ideas. I've used inspiration from both books so far, and am currently in the process of terrifying my group with a Schismatical that has gotten loose in a Logician facility they were infiltrating. I couldn't help but chuckle when the the groups Tech-Priest linked himself into a cogitator to try and work out what was going on.

From a player perspective the Inquisitors Handbook and Radicals Handbook would probably win out thanks to their wealth of extra options for origins, careers, alternate ranks and fancy kit. The background on the various radical factions is pretty nice to have as a GM though, but doesn't offer the variety that theDotDG ad the CA grant.

I've got to say that from a GM's point of view my top 3 source books are

Rogue Trader (yes i do mean it)

DotDG

RH

Why do i have Rogue Trader in there, simply because we use it as an 'expansion pack' for Dark Heresy, and with the extra careers, gear, and settings it has proved invaluable for giving my npc's a little variety (want to watch a group squerm? throw 6 of the daemons from RT into the mix and than hide the smirk behind your GM screen) as well as talking them to some truely strange places (footfall anyone?)

Anyway id like to take this oppotunity to say thankyou to FFG, you have done a magnificant job with DH and RT and I can't wait for more content from you (and as someone who has contverted the DH rules to a completely different Sci-fi setting I think i've got some idea of how much effort you guys put in bwhind the scenes).

Surak

Most used so far: Creatures Anathema

Most usefull content (even if not put to use right know): Disciples of the Dark Gods.

The Inquisitors Handbook by far. It is so cram-full with usefull stuff. We use it in almost every session and it was the sourcebook that really gave life to our DH campaign. All these weapons, gear and alternative carreers are simply brillant and it is worth its weight in gold.

Second most favourite is Disciples of the Dark Gods. Great antagonists, very fitting and ready to use for every kind of campaign.

I think Radical´s Handbook, it´a wonderful on its own right and ti offers compromise between IH and DotDG

Inquisitors' Handbook hands-down. The sheer amount of extra background paths, alternate ranks, a bundle of weapons and other gear, and sheer force of FLUFF smattered throughout it make it the next best thing to the corebook IMHO. In particular, I think the class of gear offered was perfect - enough 'iconic' stuff to up the ante a little without getting into Space Marine Power Armour and Orbital Bombardment territory.

Overall, it did a perfect job of developing more options 'sideways' than it did 'vertically' if you get my drift.

Close second comes Radical's Handbook for the same reasons, but with gear upping the ante a little bit more, and obviously a more narrowly focussed (I'd expect the same form a 'Puritan's Handbook' so this isn't really a massive grudge or anything for me). After that comes Disciples of the Dark Gods for pure fluff awesomeness more than anything else (the good mechanical parts such as sorcery and daemonweapons were reconstituted in RH so it's less important now).

Creatures Anathema gets a mostly 'meh' from me due to my PCs mostly dealing with Malleus and Hereticus territory, so all I really got was a slight "ooh" from seeing the tyranid stats. And I'm usually pretty good with generating scenarios so the premade adventures are little more than fluff libraries for me as well.

Ascension may well enter the list somewhere at the top end for me, depending on how awesome it turns out. As long as the career paths aren't anywhere near as bland as the ones in Rogue Trader (my only real gripe with that game) I should be quite a happy camper.

Disciples of the Dark Gods, hands down!

Evocative descriptions, powerful writing, imaginative and suitably horrible threats and aliens etc in there. This is the book that really brought the setting to life for me.

As a player: IH without a doubt, followed closely by RH, followed by DotDG ... yes, I am a unrepentant Radical.

As a GM: DotDG, followed by RH, followed by CA ... so many great ideas, and seeds for ideas.

Dark Heresy Core Rules - obviously
Radical's Handbook - Lots of meaty fluff and some interesting (and some disappointing) character choices.
Rogue Trader - Rules for spaceships, some extra equipment and talents.
Disciples of the Dark Gods - An excellent GM tool, if a bit long-drawn.
Inquisitors Handbook - Lots of weapons and character choices, but little more.

Favorite Sourcebook..hmmm, that is a tough question. In order for me.

1 - Radical's Handbook

2 - Disciples of the Dark Gods

3 - Creature Anathema

4 - Inquisitors Handbook

5 - Rogue Trader (Kroot and Ork Freebooters baby!)

The mainbook being the main book doesn't get a rating due to you need it anyway to play the game so its in everybodies collection.

Reason for RH to be my number one. When making a villian even if its not a radical villian....alot of those careers and alternate ranks make great npcs. For example the Heretek Savant as a go between the Logicians and another nameless group.