Mark.
I have been researching this a lot this past week and have decided that the Canon 10-18 is about the best available in the budget line. It is APS-c only.
I have read and watched over a dozen articles and tests on the Canon 10-18/10-22/Tokina 11-16/Sigma 10-20 and 12-24.
The 10-18 would be followed by Bo's 11-16 Tokina in my list. (One is for sale on the Camera thread).
The Canon 10-18 has the least barrel distortion, and Chromatic Aberration of the lot and sharpness is over the top good.
Here's a referral to a review that also has links to th 10-22. There is also a review there of the Tokina
http://www.kenrockwell.com/canon/lenses/index.htm
Here's a basic review on the Sigma 12-24 which appears to be a good lens also (I had the 70mm Sigma macro and it was outstanding in every regard, but I found the working distance to the front element was too short and swapped for a Canon 100mm macro (this is a common, and real, problem with short macros).
http://www.the-digital-picture.com/R...-HSM-Lens.aspx
The draw back is the minimum Aperture compared to the Tokina. But it has a 4stop IS system that compensates, and there is a review somewhere of this lens hand held at 1/16th of a second that is astounding. Also the f5.6 is getting near the line of diffraction limiting for APS-c but f8 is still workable. And given the depth of field on these f5.6 is still a very useful aperture.
I have strongly considered the Sigma 12-24 which is full frame and on APS-c gives next to no distortion.
Good luck in the hunt. I'm tossing up the 12-24 and the 10-18 myself. These are 19.2mm and 16mm equivalents at widest angle, which are both very wide
Trev