According to DXO, the sharpest standard zoom for Nikon full frame cameras is the Tamron 24-70 2.8 VC, edging out the Nikon 24-70 2.8G. See DXO's report here: http://www.dxomark.com/index.php/Publications/DxOMark-Reviews/Tamron-SP-24-70mm-f-2.8-Di-VC-USD-Nikon-review-An-affordable-fast-standard-zoom-that-comes-out-on-top . The Tamron 24-70 VC is also less expensive and has stabilization. I haven't tried the 24-70 VC myself but find this very intriguing.
Of course, sharpness is not the only factor for choosing a lens. Other factors include the autofocus speed and accuracy, and the quality of the bokeh, factors not taken into account by the DXO comparison. In that regard, I found the Nikon 24-70 2.8G to have fast and accurate autofocus. As for the bokeh I find it pleasant. See here: http://betterfamilyphotos.blogspot.com/2012/10/a-day-with-my-precious-ones-and-my-kids.html Hopefully the Tamron excels in those areas as well.