If you plan to ride in all types of weather, I recommend a waterproof backpack or a backpack that comes with a rain cover. I own an Ortlieb Flight backpack that is lightweight, waterproof, and it has a suspension frame to prevent sweaty back. You can read my review by clicking on the link at the bottom.

Other companies that make backpacks for all-weather commuting are Deuter (come with rain covers), Ergon (BC-3 model is weatherproof), Seal Line, Banjo Brothers, Chrome Bags, Reload Bags, Freight Baggage, and Seagull Bags. The last three are small companies that do a lot of custom stuff. I own a medium sized Freight Baggage backpack that I've been quite pleased with so far. It is massive!

Ortlieb Flight 27 Backpack Review