In a field shot comparison today (at infinity focus) on the D3x, I came away very impressed with the Voigtlander 40mm f/2 Ultron ASPH. It handily beats the Nikon 24-70 zoom, and by f/5.6 offers corner-to-corner sharpness across the frame. It’s one of my Great Value lenses on my Gear page. (If it’s out of stock at B&H, try Amazon.com, and I appreciate the use of my links—thank you). You can also get it at CameraQuest.com.
At $379, which includes the shade and a close-up lens, it’s a bargain! Add in the build quality (very similar to Zeiss ZF build), and it’s a little jewel.
The 40/2 Ultron is the smallest “pancake” lens you can put on your Nikon, which for me is a huge plus for some outings. On Nikon it’s fully “chipped” (though manual focus), meaning you can control the aperture via the camera in 1/3 stop increments. Canon EOS users can shoot it with a lens adapter, see my Gear page.