It occured to me after finishing Justice League heroes, that I agree with earlier posts the best villian for this group to face has to be Darkseid. He's the only real threat to unite that many heroes AND keep the audience's attention. Who would really care about a bunch of generic aliens invading. If an audience wants to see that then watch Independence day. And having Lex and his cronies wouldn't work either. A Legion of Doom would be pretty hard to pull off and then there is the recasting that goes along with that. But Darkseid adds a great dynamic and offers a real threat. Those familiar with the comics and even the tv series will be able to pick him out easily. Plus when compared to the more realistic Superman Returns, Darkseid doesn't fit as well. But here with all these other superheroes that suspend belief, he's perfect.