Mr. Menthol:
For me, it's right up there with the Eisenhorn Trilogy and Gaunt's Ghosts. Lots of action, of course, but the way you see Horus and the Luna Wolves drawn down into Heresy is magnificent.
I suppose though Horus' rebellion is only 'heresy' in hindsight, as the Imperium isn't really a religious institution during the timeline of the books. My favourite theme in the first couple of books was the irony of the space marines' desire to give their warrior code structure and ritual leading them into the clutches of Chaos, despite the Emperor insisting they spread the Imperial Truth of, well, atheism. Which is kind of the opposite of what they're like 10'000 years later.