I think the most well done one was the Robot one, but my personal favorite is the anime one, since anime fan lel.
Anyways yeah I think you should go for that technology look in your art work. You seem to work better with the metallic colors like the grays. The creepy one has some odd eyebrows however, no offence hahha.
With the anime one maybe you should try to transition the shading better. The shading pops out rather than transitions from bright to dark smoothly, unless that is the look you wanted. I like thinks to transition smoothly though.
The spider one was just plain odd in my opinion, rightfully so. I don't have much to say about it.
Other than that I couldn't do art for the life of me, just my opinion on it, and well done. ^_^