Tyranids of course!
Seriously though. Is there any model idea or army concept that really grabs your imagination. I've got a winged warrior squad that Im really looking forward to finishing. In between steps for the warriors, I do some of the drudge work for the army - painting gaunts and genestealers (there are ALOT of them). But its really the warriors that are keeping my interest right now.
Which ever of your armies that you feel has something that is interesting enough for you to be willing to do whatever boring stuff needed to complete it is the one you should go with.
If you want ideas about how to finish off the last 200 pts of Nids, post what you currently have and we'll offer suggestions
