For stills, the Nikon D800 has easily the best sensor available for 'normal' money. I've looked and compared a lot of pictures. There's no doubt.
However, 5D MkIII is a better camera. It also does video better.
Right now, that is pretty much the situation across the ranges between Nikon and Canon. Nikon, somewhat better sensors, for better stills-image quality, particularly at good lighting conditions, low ISO numbers. Canon, better cameras in other regards, better focusing, better handling, better quality (no oil splashes on the sensor), better service.