This is an old age question and I haven’t read all of the previous comments, nor am I going to, as I don’t want it to dilute my answer.
In our opinion there is only two(obviously their are may more and I’m am sure they are all good in there own way)
VRay (Without a doubt returns the highest quality renders within SketchUp if there is no time frames)
Shaderlight (In the real world you can’t spend days making renderings so this is the perfect option, All of the images rendered on this site are made with Shaderlight .www.fruntside.com and here are some really cool ones of a jeep wrangler http://www.fruntside.com/gopro-challenge.htm all made with Shaderlight
Best of luck with your renderings in SketchUp and if you have any questions, be sure to drop me a line
Cheers
Manu