It's a tough call. Either the Space Wolves for their tedious naming convention and general Mary Sue-ness, or Grey Knights for a few stand-out pieces of atrocious fluff (Purifiers, Draigo) and complete butchery of previously established background.
Blood Angels have the tedious naming convention as well, in addition to a few herp-derp moments like the Necron bro-mance, but overall they are no worse than they've ever been. They've always been vampires in space who were very sad their daddy died.
Space Marines has that whole Ultramarine wankery going on, but if you read it as talking only about Ultramarine successors (i.e. half of the Space Marines and growing), it's not that horrible. Although it gets annoying when Ward takes previously written fluff, and edits a few words here and there to make the boys in blue moar awesomez for no particular reason.
I'll have to agree with several posters above me that all the books are pretty much garbage with the exception of the Dark Eldar (excellent), Orks (cartoonish and hilarious), and
IG (passable).
Automatically Appended Next Post: Grey Templar wrote:. leave the actual writing to Jervis.
Jervis is an abomination, and his writing is so dull and listless as to make me want to slit my wrists rather than read any more of it.