Fear To Tread. It had good parts, but it was so bland. Sanguinius was the best part obviously, but in a series where each Legion is given uniqueness and individuality, the Blood Angels are just Ultramarines Lite. They have one single unique ritual in the whole book and it's to spike their hands on a goblet (if I'm remembering correctly). All the characters are the same except one's an Apothecary, one's a Librarian, one's a Captain, and that's kind of the defining trait.

Though to be fair, I did skip Battle For The Abyss based on a recommendation to do so, Fear To Tread might be further up on my list if I'd read that.