I vote for entry 3.
I like his use of the brick background, and the complicated text which is hard to read, but has an interesting message. The color scheme is very good, although the top right of the sig should be a bit lighter imo, since it makes the message hard to read. This entry is the only one out of all of them that I could imagine seeing out on the street. Another thing I don't like about the sig is that the color of the dripping paint is yellow, when there is no yellow elsewhere on the sig. I think it would look better dripping red. Overall it's a great sig.
Signature #6 was a close runner up for me, but I feel it doesn't fit into the theme as well as entry 3. Also I don't like the wings on it, they do not fit into the theme of graffiti at all, they look like real wings. If the wings looked like they were graffiti as well, it would have been better imo. Also I don't like the black background, it looks too plain.
I liked signature #5 as well, but it was too cutesy, and my personal image of graffiti is not cutesy. Also the image of the woman's face in the background doesn't match with the rest of the graffiti on the sig.