I don't think there is a "best" renderer. Most of them really do good. If you are experienced enough you'd get good images in mental ray or vray or final render and even in scanline (max)...It's a choice of yours.
Besides, mental ray has an ability to divide one rendering job to many nodes and there are some hardware renderers available in the market.
Vray is better in gi (imo) and final render does self illuminated objects in a better way...
I haven't tried other renderers but what i can suggest is; Quality of the scene/object/scene setup is much more important than the renderer you use.So paying more attention while building the scene is the thing that should be done.