For wide angle, I'm assuming you mean starting in the 17mm to 18mm range...
I've tested the Canon 17-40mm f4L, 17-35mm f2.8L, 16-35mm f2.8L, Sigma 18-50mm f2.8 EX DC, and Tamron 17-50mm f2.8 XR Di II lenses. In terms of sharpness and contrast, the Tamron and Canon 16-35mm results were very close...too close. 2nd came the Sigma 18-50mm, then 17-35mm, then 17-40mm. This by no means was very scientific, as I had onlyone sampling of eachlens to test...but with the copies of the lenses I had access with, those were my results. All tests conducted with my 30D mounted on a tripod, w/ remote shutter. Tests were done at f2.8, f4, f5.6, and f8.