I don't even know how I can pick one or the other they are all soo cool it's so hard to decide but if I can get cool daemon wings I think I'd be set lol.
The Iron Warriors, Night Lords, and Alpha Legion didn't even get a blurb in the 7th edition fluff-book. Was really disappointed by that. It's like they hate non-Black Legion / Word Bearers Chaos Undivided.
Word Bearers of course, the best legion (though Night Lords and Iron Warriors are very juicy candidates too.
I can understand the Black Legion being a choice or even the Word Bearers and also why the Night Lords and Iron Warriors are there, being best suited to the classes associated with the space marines and within the game, Alpha Legion being the pick of the undedicated legions for me, but to express the true power of the warp would it not be best to use the legions dedicated to the dark gods. For starters there is four and even though they're elite units in the tabletop game, they both have there advantages and disadvantages that would easily transfer across. The World Eaters renowned for close combat, The tough implacable plague bearing Death Guard shooting their way to a foe before tearing them to pieces in close combat, The noise marines of the Emperors Children bearing their sonic weapons and of course the invulnerable ranged specialist rubrics of the Thousands Sons wielding space marine killing inferno bolts led by sorcerors unleashing the ruinous powers of the warp upon the loyalist scum or whoever stands in the way. Ah well Black Legion it is for me then, sigh!
New, cult legions would be a bad idea as they do not leave room for variation, once you have your legion that is it, the options become so limited and the options to hone a warrior from another legion like the ones included in the game gives a much more rounded playstyle where one could dedicate them selves to a specific god or not at all.
they already said why they didnt go with any god dedicated legions, itd be weird seeing a world eater running around using magic, or a thousand sons sorceror of nurgle/slaanesh. and itd be unfair to limit peoples choices once they chose a legion, because some might not know the specifics of each legion, so with undedicated legions anyone can go anything. but i think they said theyll be putting god specific items/classes later, maybe as part of the tech tree theyll have, like itll say bezerker; superb mellee, increased speed, attack damage and attack speed, unable to use magic. or something similar
The cult legions will be post-launch. Anyway I would see something like witchkiller in WE instead of sorcerer