Do you want a zoom, or a prime lens? With zooms, many people are very happy with the Sigma 15-30mm lens. From the sample shots I have seen, they are sharp and have good contrast, and rival my Canon 16-35.
Sigma just came out with a 12-24 or so, which also gives very pleasing results.
Both Canon and Sigma make a 15mm f/2.8 full frame fisheye, which give excellent results, the Sigma being much less expensive than the Canon. Even when "de-fisheyed" through software, these lenses still give a field of view similar to an 18mm lens on a 35mm camera when used on a camera with the 1.6 cropping factor.