IMO, the EF-S 17-55mm f/2.8 IS is the best general purpose zoom for an APS-C body. It's optically slightly better (just slightly, though) than L-series lenses covering simlar focal lengths (16-35 II, 17-40, 24-105) when comparing on the same APS-C body, has a constant f/2.8, and has IS. Build quality is good, but short of the L-series lenses.
The 15-85mm is also worth considering - a bit more distortion and worse vignietting, variable aperture that's 2 stops slower at the long end. If you'll be shooting mostly outdoors or using a flash indoors, don't need f/2.8 for portraits/action, it's worth considering as well.