For a Wallpaper kind of work, 5 and 6 would be great.
For a concept, 1 and 3 (focused on beast), 2 (focused on rider)
For an illustration, guess 1 and 6 looks better to me.
My problem with your favorite one is the rider's head almost out of frame (1 has also a little of this feeling). 5 and 6 had strategical positions, I guess, and they convince me about the overall composition.
Actually, I'm all for the mid-left, top-left, or bottom right. I can't really choose between the three ! Sorry if that's not too helpful. Overall, they all look fantastic, and any you detail further will be greatly more so.