Best is imo Aaron Dembski-Bowden (night lords my beloved <3)
I haven’t really read any bad black library books but from what I’ve heard Ian Watson’s got some crazy stuff going on in his head.
Best and worst Black Library authors?
Extremely subjective. For me, my top authors (in no particular order) are Mike Brooks, Nate Crowley, Chris Wraight and Robert Rath. All extremely talented and evocative writers.
Worst is C.S. Goto. I think this is almost universally agreed on. Utterly ignores, if not downright disparages, established setting lore, and projects a vicious hatred of the Eldar that comes off as genuinely racist. You have to read it to believe it. It’s bad.