I will just go trough them one by one, ok?
May become a bit lenghty, so consider this the tl;dr warning
Definetly my favorite of the bunch.
The armor design looks great. I like the idea, the shapes of the armor plates you were going for and especialy the flag on the left shoulder.
It could have been improved by putting in some consideration for the human in the armor, how he moves and how the armor will reacts to his movements.
Also, the gun...
Try to paint them using a reference at first if you are unshure. Also study the mechanics of said guns. It will help you to understand why they look and work the way they do.
Granted, I am not a pro in faces myself (no kidding, screw up noses to this day), but there're problems with the proportions (the eyes look off and the nose looks dislocated) and the lighting (It makes the head look extremely flat). Lips and chin look great, however.
Also, the hand in the foreground, holding the fruit of eden (I think that's how it was called in AC) are hard to make out and rather irritating. Less blur or leaving them out entirely would have helped.
It is clear that you put alot of work into that one and it's likely that this eye is supposed to be focus of the picture. It looks gorgeous. You also got many other parts of the person very well (The nose actualy doesn't look out of place here) but the chin and lips look very pointy. This may have been a style choice, just figured I should point it out.
The teeth, however, bring me into an awkward position, because explaining this problem is barely possible without a redline. Request one and Ill do one, ok? Lets just say: Also consider your perspective for the jaw.
You may also want to use more saturated colors, but this is more a thing of personal preference here : P
#4 (Your WIP, I assume)
Looks actualy very promicing. It shows that you have a good understanding of the facial proportions, or proportions in general. (It's a more recent one, isn't it?)
I'd realy love to see the final version of this picture.
Overall: Good works so far. They have their flaws, but hey: We are here to learn, aren't we? Keep up the good work.
Until then and welcome to CA