Migo - gets my vote. Clean and solid. I don't think anything needs to be done with it. It doesn't stand out, but the layout is nice.
Billy - I don't like to see objects like that brush ending right at the edge of the screen. I like to see mountains, water, and things branching off. Also you could add a few more trees to that brush (green stuff).
Zebra - I like it, but what I don't like about it is the straightforwardness, like the straight walls and boxyness of the flooring, plus it looks, just slightly a little bit cluttered for my tastes. Also the floor color changes look odd with no transition. You're good on detail.
I PROMISE you, I didn't pay that guy to vote for me! *shot*
But really? I think my shot is much better now... still gotta fix the lightning effect on the windows though...