Canon EF-S 10-22mm is likely the best choice. It's a fine lens optically and unusually resistant to flare effects. Some of the other lenses feel better built, but none will out-perform the 10-22mm. My main complaint about it is the separately sold, matched lens hood is huge, but effective... be sure to get it.
My second choice would be Tokina 12-24/4. I've used one for a number of years. It's performance in all respects is nearly as good as the Canon 10-22mm and it actually seems a lot better built.
Third, Tokina 11-16/2.8, except that pairing it up with a 24-105 you'd have a significant gap in focal lengths, from 17mm to 23mm. The Toki 11-16mm is even sharper than it's 12-24mm sibling, and just as well made. But it's much more susceptible to flare... Plus the zoom range is super narrow, a trade-off to get f2.8, which is much less likely to be needed on an ultrawide, than on a standard or tele lens.
Also, there are two versions of the Sigma 10-20mm... a less expensive one with a variable aperture and a more expensive version with a non-variable f3.5. The latter is one of the largest and heaviest in this category.... It uses an 82mm filter, while most other lenses in this category use 77mm.
The Sigma 8-16mm is the widest non-fisheye lens made for crop cameras such as 7D. Same possible issue with gap in focal lengths when paired with 24-105. Protruding front element makes it difficult to use with filters. Such a wide lens also has inherently strong wide angle distortion effects.... For example fairly obvious bending of straight lines, though no where near as much as a fisheye.
There is also a Sigma 12-24mm, but it's actually a full frame lens... the widest non-fisheye available for FF, in fact. As such, it has strong wide angle distortions and is rather large and pricey. Would work on a crop camera, too, of course... but you are paying a lot extra for a FF lens and actually getting lower IQ than is possible with some of the other lenses mentioned here that cost a lot less.
The Tamron 10-24mm offers the widest range of focal lengths in a single lens. It's one of the most affordable lenses on this list. I have only briefly tested one and it's now been some years ago. I have heard from some users it's a bit less sharp at the 24mm end of the zoom. The Canon and Sigma lenses have faster focusing too, though on an ultrawide it's really not very noticeable... they only need to move their focusing elements a short distance to achieve focus... so even less sophisticated AF systems can serve quite well.
17-40L is a fine lens, just not particularly wide on a crop camera. When I went from shooting film (full frame) to using crop sensor digital most of the time, I sold a 17-35/2.8L and replaced it with a Tokina 12-24/4 (which actually feels very similar to the L in build, size and weight). More recently I've added the Canon 10-22mm and have started using it.
I agree with artyH, if a 17-something zoom made sense, for use on a crop camera I'd definitely get the EF-S 17-55/2.8 IS instead of the 17-40L.
But I really don't think a 17-something lens makes much sense for use on a crop camera when paired up with a 24-105L.