I think that they are all good. They all look very similar though. The bump maps are different and the colour used in the I'm guessing noise for diffuse is changed in them. I personally don't like the way your eye looks but that's just me. As for the skin texture, it's pretty good. I can try and make some general sugestions. Even though the colour of the skin is random, the texture is teh exact same for all the skin. Like if you look at a human there are places where the skin is very red, etc. You can try adding a bigger noise, and use two different versions of your skin in the two colour slots. You might want to try a falloff map, it can help create a more relistic organic texture because it helps sort of fake tranlucency. This guy knows his materials, this is something he wrote on organic materials. http://www.neilblevins.com/cg_education/complex_mats_falloff/complex_mats_falloff.htm