http://www.nikonchat.com/nikkor-50mm-lens-1-8/

nikon lenses that are best to blur the background?
Nikon – Nikkor 50mm f/1.8 AF Lens or Nikon – Zoom-Nikkor 55-200mm f/4-5.6G IF-ED AF-S DX VR Zoom Thanks
To blur the background, you want to use a wider aperture (smaller f-stop number) … The 50mm f/1.8 will provide you with the wider opening.