i know that there is a lot of love for the Tokina 111-16mm, but in the world of UWA's one mm can make a lot of difference, that's why I regard the Sigma 10-20 (or the newer 10-24) as excellent comparative lenses.
Well, objective tests generally put the Sigma 8-16 at the head of the class, and it's the widest available, by a large margin, but not filterable. The Tokina 11-16 also looks great, trading off some of the Sigma's width for f/2.8 speed, and filter capability. These are the two I'd pick from. Do you need to use filters (Tokina)? Do you want the widest possible (Sigma) or the fastest (Tokina)?
I owned the Sigma 10-20 f/4-5.6 version when I shot DX, and it was generally a nice lens, I have no complaints about the files it produced (if empirically less sharper than the aforementioned lenses). Being that it is quite a bit cheaper, on the used market, than the top two lenses, it might be the "best" value lens out there if dollars are tight.