It depends how wide you want to go. If you want to go real wide, then something like the 10-22 or 10-18 are the way to go. I have the 10-22 and I like it. Any barrel distortion is easily taken care of in Lightroom. I've heard good things about the 10-18 and it seems like an incredible value. Others also speak very highly of the Sigma 10-20mm f/3.5 and it is probably better than the 10-22/10-18 (note: Sigma also has a 10-20mm f4-5.6 that is not quite as good.)
Since you are also considering lenses above 15mm, another possibility is Canon's 15-85mm. It is my main travel lens and the extra 3mm on a crop under the 18mm makes a HUGE difference. Certainly not as much as 10mm but it is significant over the 18mm and even the 17mm. It is not quite L quality but it is pretty darn good.
If sticking with a Canon crop body this makes the most sense. The 15-85mm is, in my very humble opinion, the best all around lens for APS-C. Great range, 15mm is plenty wide, excellent (near L) IQ and great IS. The 10-18 or 10-22mm are also very good choices, but I would recommend the 15mm as a purchase and a 10-x as a rental to see if you need to go wider before buying. I own both 15-85mm and 10-22mm, very good lenses but the former fits most of my needs. Now that I also have a FF body and 16-35mm I'm selling the UWA.
Another great choice, and even though I don't need it I really want the 24mm pancake.
I don't think the 16-35mm or 17-40mm are great investments with a crop body unless a transition to FF is planned for the near future. The're both very good but not quite wide enough.