Attached Link: http://www.topogun.com

I like Topogun for retopology. I recently used it for a mesh for work that I made. The head of the mesh started out as a sculpt from a sphere and then got retopod in Topogun and later continued to be modeled in Silo and Modo. Topogun is pretty fast and version 2's beta is shaping up nicely. If you're looking for a dedicated tool for retopology, few can beat it.

 

That said, Modo, Silo, Max, etc. all have the ability these days to do retopo work. It's a simple snap and constraint action, so it really depends on how well you know your tools and if you feel like including a new app for this one thing. Topogun has some additional features like texture swapping from the sculpt to the lower res model and other map generators that may make it worth a look to you. Otherwise, you may want to see if you can find a method in your current app that works.