what are the rendering software for autocad 3d models
Could you mention what autocad program(s) you are using?
What type of models will you render?
What do you want the results to look like? Adding example photos would also be great.
How would you define "best"? The fastest? The highest realism? Easiest to use? Least expensive?...
Based on "AUTOCAD 2015, product rendering like fire extinguishrs, valves, etc."
I'd say why not give Keyshot a try? It is very popular, and many of the images on this site are done in it.
There are several other rendering programs you could use as well, but a huge advantage with Keyshot is the user base. If you have a problem, there are lots of people who could offer some advice.
All of these images are created with Keyshot: https://grabcad.com/fredswug-1/projects
What file format will you use? As Dhananjay pointed out, there are a number of formats Keyshot reads. His URL did not work for me, so try this one: https://help.keyshot.com/customer/en/portal/articles/2568560-which-3d-files-can-i-import-into-keyshot-