In my opinion : That's what I call art ..
As I 'm not a critic (just a viewer), I just want to express my admiration to these parts of every one of them :-
One :-
- the most left part of the face in the first one. ( i.e. the borders of the face , the mouth, and the nose)
Two :-
- That challenging eye look, face expression, and metallic feeling in the second one.
Three :-
- Light effects gave the third one a good advantage and also that metallic feeling of the character's wears... especially his the ones in the top part.
Four :-
- The hair is great in the fourth one, there is some miss lines in the right part and two tiny green gap (the first is in the right near the center and the second is on his forehead), but it still looks OK because of the great looking center part (i.e. the forehead part and above it)..
Another great part in my opinion is these wrinkles (or may be better to say furrows .. I 'm not sure what I can call it) any way these deep lines in the sides of the face are really great .. they gave a tangible feeling of deepness and also an emotional feeling of struggle and tiredness.
As you see I am fond of the last one .. But I think it needs some more changes to be a great artwork. Finally, Your work seems really fantastic.