XB500 has the most 'hip hop style' bass out of those choices...meaning that it's the most like a big subwoofer.
The V6 is a great headphone but it's tough to appreciate sometimes as it's fairly neutral.
Grados are the worst idea ever for rap.
The M50 is a great headphone and probably has the best impact of that list.
I'd decide between the XB500 (Or the 700), and the M50.
EDIT: As mentioned above, check out the HFI 580 as well.
So I have been looking at a lot of heaphones that have a nice bass response for hip hop and rap type genre that sounds clear and delivers pretty hard. I have come up with a list of a few I am interested in, my price range is to about 150$ including amp if it would help.
Edited by MorbidToaster - 10/5/11 at 6:56pm