What are the best softwares are available for rendering?
You have many options of software available as people have listed here:
SolidWorks PhotoView 360
Creo Elements Pro
Autodesk 3D Max
Vue 9 Esprit
And others not mentioned:
The issue is as Tomas pointed out "that you know how to fully exploit". Many of these softwares have a steep learning curve and can take weeks before you are able to produce anything up to the standard you want.
Therefore the "best software" should be one that's simple to pickup and use with a small learning curve that can produce meaningful results.
I would suggest taking a look at SolidWorks Photoview 360, or Keyshot.
I definitely agree with Matthew, pro rendering is a job in itself, sometimes even more time consuming than the conception itself.
As a long time solidworks user (for architecture!) I decided to use modo from luxology to render (photoview 360 engine). I am getting decent renders after six months of quite hard work, the mindset is different, as are the objectives.
Another factor is computation power, rendering a 500 part assy in a photo-realistic setting with daylight and shadows will shake up a CPU, and correcting a small mistake will take hours of rendering.
So ... it all depends on what you need, Beautiful work has been done in Sketchup (see their site) and much crump has been done with more reputed software.