I have the Novara Commuter backpack (it's been discontinued by REI), which has been outstanding:

Novara Commuter Backpack - REI.com

A few features that I really appreciate after using it for three years that you might think about as you browse different packs...
1) Built in raincover - I frequently get stuck in torrential thunderstorms or wet roads and this is effective enough to keep my clothes/laptop dry until I get home.
2) Built in u-lock pocket - easy access on the side lets me not have to fully take my backpack off or disturb my other pockets to get out my lock.
3) Three large pockets for compartmentalizing items - one large one for laptop, clothes, and groceries for dinner, one medium pocket to hold extra tubes, pump, multi-tool (and other items covered in bike grease), and one small one that holds keys, wallet, phone.
4) Bright color, reflective tabs, reflective piping - not sure if it actually works, but when wearing this I feel as though I am more visible to cars.