'Helsreach' is the best of the ones I've read by far. That having been said, I'll admit to having read only a few because the majority have genuinely disappointed me.
The first I read was 'Rynn's World'. Like a few other readers (from what I saw in the forums and the blogs), I was surprised by the format. Being a bit of an armchair historian, I expected something that would be kind of like an expanded, paperback version of a battle report. Kind of like a smaller scale of the hardcover BL released on the Sabbat Worlds Crusade, for instance. That's neither here nor there, though. As a novel, 'Rynn's World' wasn't very satisfying to me. I thought Parker did a fine job with the opening chapters, got mired in the survivors' rather un-engaging journey to the city, was distracted by an equally un-engaging battle on a different planet, but finally closed on a (somewhat) strong note. I'm not sure that's all the author's fault, though. Some of the SMB stories were at least somewhat pre-determined, and the author had to follow certain plot points.
'Helsreach', I thought was excellent. It had a strong protagonist with a nuanced point of view that, if not necessarily relatable, was at the very least interesting enough to suck you in. It also benefited from an excellent supporting cast that, with at least one secondary character that the majority of readers appear to have been very invested in. The pacing was quite good, the action suited the format of the story well (without the author having to sacrifice plot, drama, or interaction), and the ending wasn't just satisfying - it was apropos to the situation and the characters as well.
I never got into 'The Hunt for Voldorius', probably because my expectations had been set so high by 'Helsreach'. I've yet to finish it - I got distracted by other books - but I will.
I purchased 'The Purging of Kadillus' out of obligation; the Dark Angels are, after all, my favorite Chapter. Unfortunately, 'Purging' was a disappointment. It lacked the drama, engaging characters, and action of 'Helsreach'. I did a whole review on it (see the Reviews section), and I really don't feel like getting into it again.
I skipped 'The Fall of Damnos' because I'd been disappointed by the Salamanders novels Nick Kyme had written.
I next had the opportunity to read either 'Battle of the Fang' or 'The Gildar Rift'. I went with the latter simply because it wasn't a well-known "historical event" like 'Fang'. I hate to say it, but I was disappointed by this one as well. The plot - and the action - were disjointed, and both suffered from a secondary plot-line that really didn't offer anything to the story as whole. The author told me quite a few things, but didn't show me... and that was especially evident when it came to the tactical acumen of the protagonist. Many of the battle scenes (and especially those depicting void warfare) didn't seem to make sense in terms of how they unfolded - why the characters did what they did, and what effect their decisions had.
The last entry that I read was 'The Siege of Castellax', which I enjoyed overall, but wasn't necessarily thrilled by. I thought there were several strong and interesting characters, and the overall plot was engaging and well-paced. Unlike 'Helsreach', though, I felt that the characters were ultimately secondary to the battle. That's an easy mistake to make, I think, when you're writing a novel about a battle, but one ultimately has to remember that the battle is the sum of the actions of the characters. The Battle of Normandy, for instance, isn't so memorable in and of itself, but because of the actions of the men who fought it, and the difficulties and dangers they had to overcome.
I next hope to finish 'Battle of the Fang' and 'Legion of the Damned', about which I heard good things. Has anyone heard good things about 'Flesh of Cretacia'?