Best wide angle lenses for Canon 60D
Canon 10-22mm f/3.5-4.5 USM is a great wide angle lens for Canon APS-C cameras such as Canon 60D. Comparing with the third party lenses such as Sigma 10-20mm f/4-5.6 and Tokina 12-24mm f/4, Canon 10-22mm has more accurate focus, full time manual focus and less CA. But Canon 10-22mm is expensive ($749) even after the rebate. If money is not an issue, buy Canon 10-22mm.
Otherwise consider the options below:
Two third party wide angle lenses have better value: Sigma 10-20 mm f/4-5.6 and Tokina 12-24mm f/4. Tokina 12-24mm is sharper. However, Tokina 12-24mm has some CA and not wide enough for some people, since the extra 2 mm makes a lot of difference. If you shoot in Raw, the CA can be fixed easily in Adobe camera Raw.
If you prefer a wide angle lens with good value, Sigma 10-20 might be the one for you. But keep in mind that Sigma lenses are built by reverse engineering and might not work on a new camera body.