For anything on my back, I like Timbuk2. I've got their Classic Messenger bag in Medium and I've been using it to haul my nice dress shoes as well as slacks and a couple shirts; All with room to spare for other stuff. (I keep food at work on my desk so I rarely haul lunch or snacks for the workday during my commute).

They make backpacks specifically for cyclists too. Their bags are a little expensive (IE, my medium Classic was about $90) but it's top-quality and hand-made in the good ol' US of A (San Francisco, to be precise).