gods only know why, but i posted tyranids. bear with me, my explanation may just make sense in a moment...
most armies you see in tornies (as im sure you all agree) are power armour (marine, chaos, necron, sisters) which makes mid-strength low ap weapons great agianst them (the reaper exarchs' tempest launcher being a perfect example! yikes i hate that thing). so my reasoning is use an army that doesnt have much power armour, but is quite good against it. first thing i though of was nids.
with most armies filling up on anti-marine weapons, i take it a good nid army with god balance of big gribblies and mass hordes will be quite good. Cant really say though, because you never see enugh nid armies at tournaments.
well, i never said my choice would make real sense
