I'll give you my picks as someone who visits the book store about once a month and grabs titles of 40k books they don't recognize. Here's my list of top authors:
-Dan Abnett (Beloved by all, he both produces the most truly original 40k fiction and is the most technically competent writer of the bunch. There is a level of technical accuracy and writing he has that just makes his sci-fi feel so much more legit and real than almost any other BL author.)
-Grahman McNeil (Writes brick-solid 40k fare and creates characters and events that span several different books and series, so a lot of his writing has continuity. Has written some of my favorite novels.)
-Aaron Dembski-Bowden (Feels like newblood in the 40k universe and I think a lot of his stuff breaks out above the noise and repetition of 40k tropes with their ideas and execution. Always interested to see how ADB delivers something.)
-Gav Thorpe (One of the few old bloods left among the writers, Gav Thorpe has been writing for 40k for a long time in various capacities. Some will disagree depending on what they like: some people dislike his Dark Angels fiction like I hate James Swallow's Blood Angels fiction (notice he did not make the list) I like Thorpe's books because they feel true to the universe...although his writing style has been a lot of things over the years. His Last Chancers novels are ones that I really struggled to read but eventually won me over once I got past the 1st person narrative. I like Thorpe but he's been controversial with GWS fans due game stuff he's written.)
-Ben Counter (Maybe not the most original author in the BL but I feel like he commits to his story ideas and takes them interesting, evocative places.)
-Andy Chambers (Another of the not-quite-so-old-bloods as Gav Thorpe. Isn't as prolific as other BL authors and seems to deal with non-Imperial stuff with more regularity. But like Thorpe, Abnett, and McNeil, he seems directly plugged into what 40k is and so his details and craft really shine.)
-Sandy Mitchell (I put him on here mostly because of the Ciaphas Cain novels. He's got an easy, conversational writing style in those books that just makes him so different from the rather starchy and formal writing of most 40k stories. As 1d4chan puts it, he understands fun and how to do humor in 40k, in a way that enhances it rather than detracts from it.)
There are of course many more BL authors. Just glancing at my bookshelf of 40k novels I saw 7 more. In the end it's not that any of them are truly bad, they just don't do setting and detail like the rest of the above authors. Dan Abnett creates whole worlds and eco-systems of 40k grim dark to set his stories in, there's a context and a backdrop that makes the stuff other than the plot action worth reading.
Many, many 40k authors are inclined to just describe things vaguely due to the scale of 40k. Battleships are 'like cities in space, kilometeres long, bristling with vast batteries of macrocannons and tall spires like cathedrals along its spine" and that's it. Description: done. Pepper in a few obligatory "Space Marines do this because a special organ in their body lets them" or "Space Marines do this because of their enhanced physiology", a few "poured his rage into his swing and cleaved through 3 xenos at once, spraying the front of his armor with gore", add in a bad buy or a treacherous inquisitor or an Eldar farseer or a daemon and voila, you have a marketable 40k story.
So there's A LOT of 40k fiction out there and finding the best examples of it isn't always easy. At a baseline, 40k is 40k. If you're down with reading about Pyrrhic victories, space marines killing shit and being filled with righteous anger, quick sketches of a vast universe and starchy dialog, almost any 40k novel will suffice. However, if you want a good sci-fi novel on top of all that you need to dig a little deeper.