Wait until chaos is released, first of all. TS is going to be totally tricked out in the next release, and a whole bunch of other stuff is going to change. My advice: Wait and see. You may not even like them after they're rereleased.
Well, space marines have their traits, guard has their doctrines, and tyranids have something that allows you to create custom hive fleets. Those would be good if you want customization.
However, I'm going to give you the same advice I give everyone who asks for this kind of help: Just pick the army you like the look of. Because, after all, you are going to be assembling, painting and gaming with that army for quite a while, so you should like how it looks. Don't worry if it's the most 1337 army out there, just pick whatever one you like.
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