On the other hand, I think the Imperial Guard is massively unappreciated.
This basically but with the Dark Angels. I don't really get into tabletop (crack is cheaper) or the games but love the lore and the majority of the novels based on it. And I do feel that too much attention is put on the Adaptis Astartes and not enough on the other branchs for the Imperium. I would for example love a series on Battle Fleet Gothic (think Star Trek or Firefly in some respects but 40K). My favorite Imperial guard units are the Tanith First and only, and the Death Korps of Kreig by the way.
Tzeentch
Khorne
Nurgle
Slaanesh.
Looks like my list but switch Nurgle with Slaanesh with on the chaos sides of things my favorite legions being The Thousand Sons, Alpha Legion (debatable loyalities however), and the Night Lords (mostly because of the fantastic book series by Aaron Dempski).
You're kidding me. I knew about the whole temporally alliance (Matt Ward is something of a slur word among many fans) but was not aware that the necrons turned on their C'tan masters.