My favorite traitor legion is the Death Guard. The reason include...
1.) Nurgle is the coolest chaos god. C'mon you gotta admit any god that like being called "Pappa" is pretty cool in my book.
2.) Zombies Rock!
3.) The Death Guard have some of the coolest Pre-Heresy stories. Including Mortarions childhood and eventual attempted overthrow of his "father". The Dusk Raiders fighting against the overtyrants on Barberous (or however you spell it).
4.) The fact that Thypus, the captain of the 1st company, is the one who turned the legion to Chaos not its Promarch Mortarion.
5.) The fact that Nurlge allows converters to go crazy! (And once I figure out how to upload photos on my army you will all see what I mean.)