This is a pretty hard thing to vote on for me. Artisitically, they are all great. The artist did a fantastic job of capturing all of the Brickforge pieces that come in the packs, as well as conveying action and even some drama. As for the subject matter of each, I'd say my favorites are Zombie Defense, because I love zombie related things, and Alien Attack, because it reminds me of the opening scene of Star Wars: A New Hope. Then there is content of the packs themselves. The Fallout pack had that great GITD spill piece, Wasteland Drifter has that helmet/mask combo, and the Shock Trooper gear looks amazing in the Alien Attack pack. I really can't decide on a favorite.
Agreed, they all look fantastic! I like how they show different 'events' in chaos; but if I must choose ... I'm going with the amazing Shock Trooper gear.