Battle for the Abyss, hands down.
It was so bad that I nearly gave up on all Black Library.
I was new to BL fiction, about 3 years ago, and I was very skeptical of its validity, thinking it would consist solely of poorly-written caricatures, performing nonsensical acts, told in rage-inducingly bad prose. I started with Horus Rising, and was stunned that tie-in fiction could evoke such grand concepts, like the Greek Tragedy-thing of the Imperium-that-could-have-been, and that it could make toy soldiers so damn readable.
Galaxy in Flames tested my resolve a little, being a significant step-down from the first 2 in the HH series. But Abyss was so excruciatingly bad that it negativelty affected the previous books that I had read and enjoyed. It was like the scales were falling from my eyes and I could now truly see how crappy all IP tie-in fiction was. It was just cheap pulp nonsense for people who hadn't read anything deeper than a tabloid newspaper. Why was I wasting my life reading such crap?
I took a break from BL to read some decent stuff, like The Spy Who Came in From the Cold. Tentatively I started again with BL, and haven't looked back.
I have read too much excellent BL fiction now to let a bad book throw me off, but other clunkers include Outcast Dead from about p85 onwards, Angel Exterminatus and Vengeful Spirit. Feat to Tread was disappointing too.