I think my favorite so far is either the first one you did or the second one. First one seems to have the most dynamic and thought out lighting and the second one has the nicest colors.
What I think the pieces are lacking is lighting. Could just be me but it seems they are getting equal light on all parts of their bodies. It just isn't reading well.
I made a quick edit to give you an idea of how you could maybe add a bit more interest to them with some simple lighting.

Also the anatomy on the big guy seems a bit flaky.

Seems a bit stubby and thick. Not sure maybe someone can do a more in depth anatomy critique.
Looking good so far
