rap vocals are no different than any other. but maybe you might want to try an old shure 545 used if you can find them that the great jh used.
or a shure sm7(expensive). or a cheap cad or mcasp1 if you want it breathy and intimate.
ive recorded several rappers/groups. i would say its more the engineering than the mic.
see if you can tell the mic used on this. its a comical song a lot of folks like
soundclick.com/bmanning the tune "limeyrap"
peace.