Yeah, 4, 5 and 6 all seemed like the best to me and they are the top three. Number three looked like "lol Nathan Drake".
I think 6 had more potential but it looks a bit muddy. It's my favorite composition but the details and coloring are a little lacking. Perhaps I'm just not fond of earth tones and prefer sharper lines and colors. 4 is executed very well with the red and black line but it priortizes the world over the characters, to me it seems more like a cool poster or art-book page then a cover. It has a nostalgic quality though, one person commented Jules Verne, and it immediately reminded me of FFVI promotional art with Terra riding on the Magitek armor.