World eaters death guard Thousand sons and actually red Corsairs, simply bevause the tyran of badab is awesome
Sticking with the theme of Chaos Undivided, it'll likely (and should be) the Alpha Legion. Unless them, then either the Crimson Slaughter or the Blood Gorgons which are GW's new babies, both of which were featured with the new sixth ed.
Death Guard, World Eaters and Thousand Sons. In that order. I will cast my vote so that even one of those legions have a better chance to make it into the game over any other warband or legion.