As for Dan Abnett, he gets credit for kicking off the series and really sort of setting the tone going forward, but his contributions to the series have been pretty meh. Aside from Horus Rising, he wrote Legion, which I initially thought was good but now I sort of hate the way they write the Alpha Legion, Prospero Burns, which was interesting seeing all the info about the pre-Heresy space wolves but ultimately had almost nothing to do with the title of the book, and Know No Fear in which he managed to make one of the most important battles of the entire civil war be completely bland and actionless. Abnett is amazing for Gaunt's Ghosts, Eisenhorn, and Ravenor, his Heresy books have been less than stellar.
The three powerhouse books of the series so far (I'm behind and haven't read Angel Exterminatus or Betrayer yet) are definitely Fulgrim, A Thousand Sons, and The First Heretic, written by Graham McNeillx2 and Aaron Dembski-Bowden respectively. If Betrayer is anywhere near as good as The First Heretic, I'll definitely have to check out the Night Lords series. I also own Helsreach, his novel in the Space Marine Battles series, which was quite good.