Favorite: Enforcer by Matt Farrer, followed closely by The Emperor's Gift by AD-B and Prospero Burns by Abnett. I freely admit that none of them are perfect (Enforcer probably the most flawed, in plot and pacing and POV jumps, etc--but counterbalanced in so lovely a manner by the sheer 40kness that it oozes; The Emperor's Gift for a slightly lackluster depiction of the Wolves [except for Bjorn], and a general anticlimax by the ending, since little could top Armageddon; and Prospero Burns for the padded beginning and rushed ending), but they are stories that cleave most truly to the breathing heart of 40k, in my mind. WHF-wise, I'd have to go with Wine of Dreams as my favorite, followed by The Burning Shore.
Worst? I could rattle off a chain of names most people here would know and agree with, since we've all heard the horror stories of those poor souls that read them--Goto, Sons of Dorn, etc--but the truth is, I've steered clear of those particular books. I'd say my least favorite BL book I've read is... Ghostmaker, for that steaming atrocity that is the framing tale.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!