Definitely the blue head. It's much more menacing, with it's dead, blue-grey skin tone. Plus, it has this expression that's clearly craving flesh. 90% of the zombies in our zombie games have this head. I've collected about 40 of them just for this purpose. You can see various shots here
. The grey heads are a good second to add in and lend some variety to the mix.
The green head has this dopey expression on it and it's palor isn't exactly spooky. The zipper in back doesn't exactly say "zombie" either. They make good lizard heads...