First off, you need to let us know what type of Canon camera you are using. Why, because there are two types of sensors, thus two types of lenses, and the crop factor. For a Canon cropped sensor camera, you can do no better than a Canon EF-S 10-22mm USM Lens. It is like a poor mans "L" quality glass, but without all the seals for moisture and dust, and can be had for $500-700 for a copy. Now if the camera is a full framed 35 mm sensor camera, you can really get expensive with "L" quality glass, or less. Look for a lens which is smaller than 50mm, like 35mm, 16 mm, or smaller, but not a fisheye. I just took delivery of a Canon EF 16-35mm f2.8L USM lens, it is crystal clear. The full frame lenses used on a full frame camera, will be exactly what the mm is noted on the lens. On a cropped sensor, you must multiply the lens value by 1.6, to get the true value of the lens (70mm = 112mm) but this does not give you any magnification of your subject. It only give you a wider angle of view. Also, note many of the wide angle lenses do not offer IS, or lens stabilization, since most wide angles are used on a tripod. Canon has just announced a Canon EF 16-35mm f4L IS usm lens. So, Canon must see a market for this lens. Note, it is a f4 lens, and will take one stop more light than the f2.8 version.
