Imho, Rob is spot on. The 17-40 gives a nice wide FoV at 17mm, and is just starting to hint at long at the 40mm end, when on a crop body.
Ultra wides are all well and good, but imho, they just promote the "ahh b****r it, just get everything in and sort it later" attitude. They have their place, yes, but in my experience, those who consider their compositions 'tend' to not use UWA lenses.
Sure the other lenses are good in terms of image quality, but for landscapes, when you're shooting at F/8-16, nearly all lenses will be acceptably sharp then. If you cant stretch to the 17-40mm, look at some of the other canon lenses around the 16-18mm length at the wide end. I don't know any specific lenses off the top of my head, but I'm sure others will.