Photozone have done a review of the Sigma 12 - 24mm MkII and it's not fantastic. Problems especially at the wide end with border & corner resolution, vignetting, chromatic aberations, and barrel distortion, which don't really become acceptable until f/11 on top of all this it's as expensive as the 17 - 40mm f/4 L, and you can't use filters on the front. I have owned the MkI version of this lens and it was not commercially useable until well up the zoom range.
The Nikon 14 - 24mm f/2.8 is perhaps the best UWA zoom available so it's a good choice if image quality is important to you. It's manual focus, but that's not really an issue with the depth of field available from short focal lengths.
The 17 - 40mm f/4 L is cheap enough for an L lens, but it's not good wide open at the wide end, and could really do with being updated, again you will have to use this lens stopped down heavily if you want decent image quality. You can use filters with it though.
The 16 - 35mm f/2.8 L is double the price of the 17 - 40mm but doesn't give double the performance! Is a 2.8 aperture important to you? You aren't going to be able to get depth of field effects with a lens this short so it's only going to be of use in low light situations.
Of course there are other options but the cost! the 14mm prime has been mentioned but it is mega expensive, then there are the tilt shift lenses the 17 & 24mm again pricey but not as bad as the 14mm! they do serve a purpose, and suffer less from vignetting & resolution fall off than normal lenses, the only downside is the cost.
