By far the best (sharpest, lack of CA) is the Tokina 11-16. Next I would say is the Sigma 10-20 and then a firm last is the Canon 10-22. Plus the Tokina is the fastest at f2.8.
For full frame you'd have a few lenses to consider but for APS-C there is only one choice, in my opinion
EDIT: Of course weigh what others say as well but if you do decide to go for the Tokina I've just discovered a friend at work is selling his as he's just got a 6D and doesn't need his APS-C lenses anymore. Let me know if interested.