The Red guy's the shit
The one on the right is by far the most well done of them all. Did you get inspiration from the Crysis suit for him, because that's the first thing that entered my mind when I saw him. Also his head seems to be matched with his body the best. Prolly helps that he gives off this emotionless aura so that the emotionless head doesn't clash with the rest of it.
The mech is freakin awesome. And then I look at the head and I just laugh. This wee little adorable doggy head poking out of a badass mech is just too damn much for me, hahaha. Personally, I think the mech would look cooler if it had a mech head stylized after a wolf's head instead of having the pilot's head sticking out. Also, I'm curious if you got inspiration for the mech from the Tau from WH40k? It seems very much done in their style.
The head on the black, white, and gold one bother's me the most. Outfit is very suave and badass, and I'm curious as to what that cord running to the knife does, but then the head is just so... meh...
The one on the left I just don't care for very much. Once again the head is so emotionless and awkwardly attached... I mean, it's well drawn, I just don't care for her? style. (I'm also assuming the second from the left is a female also?)
So, awesome work on the outfits, armors and tech. Poses are pretty nice, as well as background and writing. Just the heads are what are really bugging me.