Depends on how much cash you want to spend and how wide you want to go?
Sigma is silly-wide but you will get better quality with the nikon 17-35 or 17-50, although both of those will set you back double the price of the siggy. The 17-35 is full frame, professional grade stuff so will last long enough and be compatible with Nikon's cheaper full frame cameras, when they eventually appear.
Good glass should last you a lifetime!