On a budget? Cheapest probably the Samyang 14mm f2.8. Really sharp, though manual focus (double check that the focus distance on the lens is correct and just do hyperfocal distance) and quite a bit of distortion (though there's Adobe Lightroom lens correction for 1 click removal)
Haven't personally used the Nikkor AF-S 18-35mm f/3.5-4.5 G ED but heard good things about it, it's probably the cheapest ultra-wide zoom with good quality from Nikon.
