well there are zoom macros and prime macros. Prime macros are more prevalent and tend to give you a better image quality. Within prime macro lens of course you have different focal lengths and minimum focsuing distances which will dictate the max magnification spec.
The macro lens of nikon that I am familar with are the 105mm VR and 60mm. VR is useless in macro. The 60mm is one the sharpest primes that nikon has ever made.