I think the better question is "what is the best camera". the camera - CCD versus CMOS - Night vision, Water proof, Minimum Illumination, size, mounting options.. I think all navi capable unit will give you plenty of quality at the head unit you want. The real weak link is the camera.
I vouch for the one I used in my backup camera thread.. CCD, water proof, tiny, small footprint, swivel mounting options, etc. etc