As others mentioned, the 14-24 is indeed the best UWA available for any DSLR. That said, it's a very specialized piece of glass that's very heavy and doesn't take filters unless you want to spend another $400+.
If I were you I would look into the Nikon 16-36 f/4 which is a great lens that's half the weight and take filters without compromising much quality. If budget is an issue look into the Tokina 16-28 or Sigma 12-24. Actually, I had the Sigma 12-24 for $350 and it's the widest lens available on FX!