Battle for the Abyss
Although I have to say, it wasn't completely bad. Every page was interesting in its own way. It's just that it seemed to drag on forever, and the plot - otherwise having good potential - was rather wasted.
Fallen Angels came close, bar the actions on Caliban, and the last page goes a great deal in redeeming the book.
False Gods, despite being a good book overall, makes it to third worst in the series, in my opinion. It's quite good, I'll say again, but the way Horus' fall was handled is severely lacking. And that's what I was looking forward to when I set my hands on it. The fact that Garvi Loken is a good guy, and a rather straightforward one at it, doesn't help things.
Fulgrim is yet another good book that I think was somewhat ruined by a poor choice of plot. The sword was a cheap excuse to not tackle deeper issues with the primarchs, such as character flaws, if you ask me... I find it rather sad that this, along with False Gods are both written by McNeill, who clearly can do a good job at character books...