what is the best rendering software for mechanical design?

It all depends on what you what to get out of your renders?

But I think the best is Keyshot
easy to learn - fast and get great renders

this is a very frequent question:

http://grabcad.com/questions/best-rendering-software-for-beginner
http://grabcad.com/questions/program-for-rendering
http://grabcad.com/questions/need-a-good-and-easy-rendering-software

I would say the best is the one which you feel comfortable with.

The type of work you do will determine what 3D modeling software will best suit your needs. There are many modeling programs and rendering engines. You will have to be more specific with your question.

Engineers might use Solidworks or Inventor.
Architects might use Revit or Autocad Architecture.
Designers might use any number of different programs depending on what they're designing.

The type of rendering software is determined by the modeling program you're using. Not all rendering software can be used with all modeling programs.