Canon EF-S 10-22mm is simply the best of the ultrawide zooms for crop cameras, IMO, regardless of manufacturer. It's currently on sale for $600 ($50 mail in rebate).
The EF-S 10-22mm is sharp edge to edge and well corrected. It's also likely the most flare resistant of all the UWA zooms. Though I do recommend using the matching lens hood, anyway, although it's rather large and sold separately. USM focus drive is for all practical purposes instantaneous, as well as accurate and quiet. The lens uses a 77mm filter, which is nice if using other lenses with the same size. Doesn't have IS. But, seriously, does it need it? At it's widest setting, I can handhold it at 1/30 or even 1/15 with little problem.
The EF-S 10-18mm STM IS is a top value at about $300. I haven't used one yet, so can't comment much on image quality. Some say it's not quite as sharp and well corrected as the 10-22mm. It's definitely more lightly built (plasticky). STM focus drive is quiet and smooth, though likely not as fast and accurate as USM. However, on an UWA that only needs to move it's focus group a tiny amount and has huge depth of field, that's unlikely to be a problem. IS is a bonus on this lens... maybe not really all that necessary, but it also can't hurt. The affordability of the EF-S 10-18mm is a game changer. Up to now, there were few UWA offered by anyone for under $450. Not I would guess the 3rd party manufacturers are going to have to lower their prices, to remain competitive.
IMO, the Tokina ultrawides are the second choice, after the Canon. It's now discontinued, but the 12-24/4's image quality and flare resistance are close to the 10-22mm, and the build quality is even better than the Canons. The Tokina feel very "L-like". All the Tokina use a micro motor focus drive, but it doesn't seem to matter much due to how little movement is needed and the inherent depth of field of very wide lenses. Tokina also rotates the focus and zoom rings opposite direction to Canon lenses (same direction as Nikon lenses)... though in use it's a minor thing. They also use a "focus clutch" mechanism to shift AF on and off... what this means is that you can't manually override auto focus (you can the Canon 10-22mm with it's USM)... have to switch the lens if you want to manually focus it.
The 12-24/4 saw two versions (not much diff in the Canon mount... significant difference in the Nikon mount where an in-lens focus motor was added to the second version... AF-S style). The 12-24/4 has just recently been superseded with a new 12-28/4, which I haven't had a chance to try yet. 12-24 are getting scarce new on store shelves, but have been popular lenses and can be found used.
Tokina also offers the 11-16mm, which is the only ultrawide to offer f2.8 aperture, for about $525. If anything, it might be sharper than either the 12-24 or the Canon UWAs. But, to get f2.8 there's a trade-off... the 11-16mm costs more has a very limited range of focal lengths and is one of the most prone to flare effects. Do you need f2.8 on an UWA? I don't. But it is popular for astrophotography and other night photography. Tokina also makes a 10-17mm, but it's a fisheye lens for crop cameras, with the usual strong effects associated fisheyes.
Sigma offers a number of UWA choices, too. They have two versions of 10-20mm: a cheaper one (about $450) with variable aperture and a much larger, heavier and more expensive one ($650) with f3.5 aperture. This is the only UWA for crop cameras that uses an 82mm filter.
Sigma also offers the widest of the wide, an 8-16mm UWA that's not a fisheye (though it has some pretty strong ultrawide effects). It's also fairly expensive at $650, but if you need extremely wide, this is your lens! It has a protruding front lens element and a built-in lens hood, so standard filters cannot be fitted to it.
Finally, Sigma also offers a 12-24mm... but it's actually a full frame compatible lens... the widest non-fisheye available in fact... But at $875 it's a bit of a waste of money for use only on a crop camera.
I compared the variable aperture 10-20mm with the Canon, Tokina and Tamron offerings and although the build felt good (better than Canon, not as good as Tokina), I didn't think it's image quality was as good as either the Tokina or Canon. It wasn't as sharp edge to edge and was more prone to flare. However, I think I compared the first version of the Siggy, which has been through one or two revisions since then, including getting HSM focus drive (similar to Canon USM). I've never had opportunity to try out the f3.5 version or the 8-16mm or Siggy 12-24mm.
Tamron offers the 10-24mmm ($500), which was the broadest range of focal lengths available in a single lens until recently (the Toki 12-28mm's range covers the same, altho it's a different range). I tested one years ago... as well as the Tammy 11-18mm it replaced. The 10-24mm's images are fine in the 10 through 20mm range, but I felt it got a little soft in the 21 to 24mm range. It also doesn't handle flare as well as either the Canon 10-22 or Tokina 12-24. It's a bit plasticky build, too. Though to be fair that might just be about the "feel", doesn't seem to effect durability or have any significant bearing on other performance factors. The Tammy 10-24mm was a definite improvement on their old and long since superseded 11-18mm, which left a lot to be desired.
The differences between all these lenses really aren't huge. They all are pretty capable and a few have special features that might appeal to certain users. I have and use the old Tokina 12-24/4 and the Canon 10-22mm. Still feel the Canon 10-22 is the best of the best in image quality and performance, and the Tokina feels the best of the bunch for build quality while coming in a close second place to the Canon in terms of image quality.
I use standard B+W filters (which are slimmer than some other brands) occasionally on both the Canon 10-22mm and Tokina 12-24mm. I have not seen any sign of vignetting with those on either lens, even with a deeper C-Pol filter and at the lens' widest settings.
Canon 10-22mm at 10mm and f11 with standard 77mm B+W Kaƫsemann C-Pol filter (image was cropped slightly for composition purposes)...
I do recommend getting the hood for the 10-22mm, even though the lens is pretty flare resistant. At some extremes, the hood will prevent any flare that can occur. Here are a couple test shots I did without and with the matching hood...
The Canon hood is rather pricey at $32. If that's too much, Vello makes a clone for $16 and there are other clones that are even cheaper.
If I sound like I'm harping upon all these lenses propensity to flare, that's because it's often an issue with ultrawides.... They cover such broad angle of view, you'll often have some specular light source or something else that's potentially flare-inducing in the image. Necessarily shallow lens hoods are only able to do so much, too.