Hi guys,

To my personal experience, Autopo and ZRemesher are very close to each other, but my preference leans slightly towards Autopo's results.

Instant Meshes is very good at following mesh flow, but is plagued by triangular dead-ends that make ugly knots in the quad flow when subdivided.

More info and a test here:

http://www.metinseven.com/blog/Ag4W/3d-coat-vs-zbrush-auto-retopology-comparison