Best Rendering tool?

what do you think it is? the internal blender engine? yafray? Radiosity? POVray? or something else?

for what?

speed? realism [though that is more dependent on skill than renderer]? abstract?

Radiosity doesn’t belong in that list. It is a feature of the internal renderer.

I don’t know much about Povray except that it has been around for a while, it has a Scene Description Language, and it never crashes.

As for the others, Blender Render and Yafray, each has different features that the other doesn’t. Yafray has caustics and GI and can produce nice images. But Blender was built around Blender Render so all the options you find in the program will be supported. It is easier to use the Blender Render and both can produce great results.