At various points I've owned a sigma 10-20, sigma 12-24 and currently a tokina 11-16.
Sigma 10-20 was great but had to filtered carefully to avoid vignetting, nice lens, compact, cheapish if you go for the f4-5.6 version.
I used it on a 400d and recall little fringing, sharpish corners etc.
I got an Eos3 film camera and so wanted a ff lens, i got the 12-24, which i loved on my eos 3 and liked well enough on my 7d, it was an extreme lens with major caveats, filtering being the most obvious, and its expensive in mk2 guise. A no brainer for ff uwa users, didn't make as much sense on aps-c
Finslly, my current uwa, the tokina 11-16. I needed the f2.8 for video use, which makes it the only lens of its kind for my 7d and 600d, pricier, you will need to defringe in PS now and then, but a sharp lens with even illumination. AF not fast, but on this fl doesnt need to be.