1) 4th Ed Nids: I liked the reports from the Imperium, and reading about the desperate struggles of Inquisitor Kryptman to stay ahead of the swarm.
2) 5th ed Necrons: I like the background for the special characters, particularly Zandrekh and Obyron. It probably helps that I never was wild about the C'tan and the Old Ones being the source of everything interesting about the current races.
3) Orks, in general the one I'm most familiar with is 4th, though I'm sure the others are even more crazy. In 4th, I liked the tale of the Demon hunter, cutting a swath through the Eye of Terror because demons were good fightin'.