What do you want from a renderer?
Photoreal quality, ease of use, fast rendering, cheap/free?
So, the answer to "which is the best renderer" is: it depends on what you need.
From the sound of it you seem to be new to rendering so I would suggest a renderer that is easy to use and cheap to get you going.
If you want it to be free I suggest Kerkythea.
Cheap and easy to use, my suggestion is Twilight (it's basically Kerkythea but directly from within SketchUp).
Photoreal (advanced), then it's Thea or Vray.
If you go for one of the more advanced you should be prepared to put quite some time into understanding how to use it fully.
There are lots of more renderers and I'm sure others will fill those in.