I'd say my personal favorite is the Soul Drinkers. A derivitive of the Imperial Fists chapeter (followed after the Crimson Fists and Black Templars), they are very similar to the templars in that they are a fleet based, ship to ship, boarding action, assualt army. Plus; I've read the books about them. Good stuff. And my chapter, the Reapers on High watch. My fluff is secret. Suffice to say, they came after the Scythes of the Emperor. Yayness. I love assualt armys, but play with shooty ones. Also, my favorite traitor legion is the Iron Warriors. Seige lords ahoy!
Pokemon are like zombies that breed... when you think you've finally seen the last of them, they come back, more than before and just as creepy.