Definitely the best IQ in an UWA is the Canon EF-S 10-22... but of course it's also the most expensive. It's a bit more "plasticky" than most of the third party UWA lenses, but doesn't seem to be any less durable. It is one of the few lenses in this category that has USM focus... But to be fair that's hardly needed because UWA lenses only need to move their focusing elements slightly to achieve focus, and their inherent depth of field hides minor focus error, so non-USM lenses AF performance is virtually identical to USM, in the field. Late afternoon, Pigeon Point Lighthouse
Most of the UWA's on the market actually are pretty good in terms of sharpness... One thing that sets them apart is how well they handle flare, which is a common issue to have to deal with when using a lens with a very wide angle of view. The Canon lens is unusually good handling flare.
Your other choices:
Sigma 10-20 with the variable aperture... The copy I tried wasn't very good dealing with flare, but otherwise was okay. I have to note that Sigma has revised this lens, so it might be better now. It's one of the cheapest.
Tokina 12-24/4... I found to be second only to the Canon in flare handling, feel it's better built than the Canon, and it's one of the least expensive models. There is a noticeable difference between 12mm and 10mm.... but 12mm has been wide enough for me. This is the lens I bought and am still using, the first version. The Toki 12-24 that's selling new now is a second version, which in the Canon mount supposedly got improved lens coatings (maybe it handles flare even better, I dunno... the changes in the second version for Nikon mount were much more significant, but Tokina seemed to just update the lens in all mounts at the same time). I wanted a fixed (non-variable) aperture, too... and at the time Tokina lenses were the only ones that offered this.
Tamron 10-24mm... Is the widest zoom range of any of the UWAs and sells for about the same as the Sigma 10-20 with the variable aperture, making it one of the least expensive choices. I haven't shot much with this lens, so look elsewhere for more info about it. I have heard that it tends to be a bit soft at the 24mm end... maybe that can be improved by stopping down a bit. I had more opportunity to compare the earlier Tamron 11-18 and wasn't impressed with that lens.... It was a bit clunky and IQ was pretty sketchy.
Sigma 10-20/3.5 (fixed aperture) is a relatively new lens that I haven't tried. It's considerably more expensive, close to the cost of the Canon 10-22. Note that it's the first UWA with fixed aperture available from a manufacturer other than Tokina.
Tokina 11-16/2.8 (fixed aperture) is the fastest lens in the UWA class, the only one with f2.8. It's one of the more expensive models and is a bit prone to flare, but is quite sharp otherwise. The tradeoff to get f2.8 is the very narrow range of focal lengths (along with more susceptibility to flare). Other folks might have reason to want it, but personally I don't see much need for f2.8 on an UWA lens. f4 is fast enough for me on an UWA, especially considering the trade-offs to get f2.8.
Sigma 8-16mm is a very new model and the widest of the UWA lenses available for crop camera. I haven't used it.... It's a little pricey, and be aware that there's a lot of wide angle distortion with such an extreme lens (though not as much as with a fisheye).
Sigma 12-24 is actually a full frame lens... the widest one available in fact (short of a fisheye). It's known to have fairly strong distortions... But, hey, what do you expect with such a wide lens? It's been revised fairly recently, but I haven't used it and can't comment about any improvements. Because it's FF capable, it's also the most expensive of all the lenses mentioined here.
You can see images made with all these lenses in the Lens Sample Archive subforum linked to this forum. Just do a search there, for any particular lens you might be considering.
Below is an image made with my Tokina 12-24 (first version) where I was deliberately trying to cause flare and other issues... I was doing a lot of things "wrong" to find out how the lens handled it...
Tokina 12-24/4 lens at 15mm and f10, with B+W MRC C-Pol. EOS 7D camera at ISO 200, 1/640 shutter speed, handheld, available light.
Yeah, there were a few things I needed to fix in this image with Photoshop.... In particular, there's some flare in the middle and chromatic aberrations near the edges, along with contrast/color saturation and the exposure level (I had a bugger of a time getting that even close for some reason, but that made the "test" image even more stringent).... But I actually was very pleasantly surprised how little I had to fix, how well the lens handled a "worst case" situation. Below is the image before much of anything was done to correct it...
A detail that shows the flare artifacts more clearly...
And another detail that points out some of the CA that I needed to correct...
It's only an Internet resolution image, but maybe this can give you some ideas what to watch for in other lenses.