Quote Originally Posted by Kayaker72
regard the 15-85 and the 17-55 f/2.8 as optically the best EFS lenses.

I agree, and I'd add the EF-S 10-22mm f/3.5-4.5 lens to that list as well (though as an ultrawide it's not suitable as a general purpose zoom). All three offer essentially equivalent image quality to many L-series lenses, although the have lower build quality.