We don't get a lot of signatures around here...
I'd say the first one is the best because you make good use of the colors and the light on it is better than it is on the other ones. There is a sense of flow in it, which I'll just assume was intentional, so that's a thumbs up for you :up:
The Paramore tag isn't very nice because the light is messed up (what's the deal with that big blotch in the corner) and I don't quite like the filter (which vaguely looks like Topaz) the render has on it. Furthermore the text is rather poor.
The third piece (with the ninja) relies way too much on C4D renders and I feel that you've done very little with the stock. I know the stock, I've used it myself back in the day.
The last one is pretty good, but that's a very feeble attempt at recreating natural light. Also some of it is too random such as the signature in the top left corner and the one at the top (middle). Also the grunge brush you used for it is very harsh, and as such clashes with the render.
Keep it up though, there's some good stuff in there. The fact that you try to pay attention to light already says a lot