For me, I'd have to say it was The First Heretic in the Horus Heresy series. I say that as someone who put off reading that novel for the longest time because I wasn't really interested in a bunch of religious zealots like the Word Bearers. So I read every other HH novel that was out to date at that time, which was around the time of Battle for the Abyss kind of dreading the point when I'd have no more HH books to read but that one. So I went into it with a more than healthy bit of trepidation and despite that it just blew me away. It does such an amazing job of humanizing the fall of Lorgar, from being humiliated in front of his legion by The Emperor to going on his pilgrimage to the legion's eventual descent into Chaos.
However, if you want something a little more one-off, I'd suggest either Cadian Blood or Emperor's Gift. Helsreach wasn't my personal favorite, though I did so enjoy the way he played Andrej and Grimaldus off one another and I don't want to give the impression it was a bad book, it just didn't grab me quite the way his other works have. His Night Lords trilogy started off a touch stiff, but got better as it went along.