Grey Knights
The Grey Knights are a bunch of fucking badass SPESS MEHREENS that also, like their brethren, do it for the EMPRAH. Playing a Grey Knights army is like playing 40k on "Hardcore Mode". In an average 1,000 point match, you'll have 20 models and a land raider. But it's okay, because they kick ass. Plus all of their models subscribe to the Rule of Cool. Are only sent to the fight only when other marinefags get their asses raped by mutants, heretics, and forces of Chaos. Usually they get the shit done, as evidenced by the continued existence of reality.
They are the testube babies of the Empra, and are thus, in a way, miniature primarchs.