My choice is the 105mm f2.8 VR. I also find it much handier to use with a 1.4 Teleconverter making it effectively a 150mm. Better working distance with small critters. I'd recommend this approach.
I use the Sigma 105mm 2.8 EX 1-1 macro. It's the sharpest lens I've ever seen (that includes the Nikkor 2.8 pro lenses). It was (if I recall correctly) about £350 - 2nd hand but pristine.
The best lens is going to depend on what you're shooting, how you want to work and the camera you use. For copy/animation stand work, the 60mm MicroNikkor on a full frame or the 40mm MicroNikkor on a half frame body.
I've used the 60mm at zoos where the subject is very close to the glass for example. But, outside of copy work, working distance is too small for most purposes.