No Forge World Horus Heresy option?
Having most of the books on the list, I can say that I pick up
IA 9 and 10 the most. Mainly to flip through and read the generals, read the blocked-out short stories scattered through the book that I've missed, and the Badab War campaign rules (that my group uses).
I flip through the
HH:Betrayal book more, and I think it's their best book, but that wasn't an option in your poll.
IA8: Raid on Kastorl_Novem: from a story perspective, it's a little weak. Classic example of poking a sleeping bear with a stick. The orks were fighting each other and then along comes the Imperium to give them a rallying cause!
From a usefulness perspective, it gave us alternate STOMPAs, The Mega- and Mecha-Dreads, and a new Ork Army list. It looks fun to play, even in 6th edition. I really don't care for the Elisian army list, but I do like their dune buggies.
IA5-7: Great books. Army lists for Death Korps of Krieg (along with what are tied with
FW's Hazard Suit Cadians for my favorite NEXT ARMY models), alternate Renegade lists for 3 Chaos God armies, along with a handful of special characters. The current allies system with
IG + Chaos Space Marines does this much better, but it was all we had after the Lost and the Damned were removed. I'd rate Zufor as a more useful Khorne
IC than Kharne! There is also an Alpha Legion
IC, which is the only one, I believe. Also, it's an epic story of damnation and more damnation.
IA11: The story was OK, but after reading
IA9-10, nothing was going to stand up to it for sheer epic-ness. This book does 2 things for my play group: It gave our Space Wolves player a unique
IC that he uses all of the time, and it gave our Eldar player a new way to field is old-ass army. He likes the Corsairs list, and it's a devil to play against!
IA9-10: The Badab War. I'm more familiar with this one than the others, having read through it the most. Benefits:
* Our group uses the Badab War campaign exclusively now. It offers a great foundation that you can tailor with your own special missions and rewards for winning various rounds. This is what Planetary Empires should have been.
* Story: Best of the
IA (non-
HH) stories. The rise and ultimate fall of Huron, whom I truly believe was doing what was in the best interests of the region and (ultimately) the Imperium. Along the way his hubris got the better of him, and he lost his perspective and rationale. The final condemnation of his legion, as well as the salvation of the Chapters that realized their mistake late in the game, is what makes this a great story.
* The Tyrants Legion. It's on the very long list of "someday I'll do this list!" Again,
IG allies + Space Marines do this list better, but rogue apothecary squads stealing the opponent's Gene Seeds just sounds so...naughty to play!
*
HQ's. Never has more Space Marine
HQ options been gathered in one place. I particularly like the Huron and the Salamander Dreadnought (Ashmantle?)