I will have to throw in another vote for the sigma 17-50 f/2.8 OS. It is definitely one of the sharpest lenses Ive used for crop, even wide open. It is as sharp if not better at some lengths than the canon 24-70L mkI, on crop sensors, and the OS is useful sometimes. My only issues with it are the lack of full time manual focusing, slightly cheap feeling build quality, and the bokeh, which sometimes just looks horrendous.