Any backpack that will hold all your commuting "stuff" and fit on rear rack or in a basket mounted on the bike is superior to ANY kind of backpack carried on your back, shoulders and/or neck while riding.
+ 1 on the Ortlieb Packman Pro2 mentioned earlier.
Looked at lots of great packs when I bought it, but in the end had to go for the totally waterproof pack. I think it is great to be able to forget about the possibility of anything getting wet even in the most intense downpour. Feels pretty comfortable as well.
There is no "best" backpack, there is only best if the pack fits you comfortably, will handle what you carry without problems, will allow air to pass between the pack and the back. Personally I don't like backpacks, but for short commutes their fine. Bike shops sometimes sell backpacks for cycling but unless they have some sort of system for raising the pack off the back a bit for air to get between the pack and your back then it doesn't matter if you get it from a cycling store or a camping store.
This is the best advice you are going to get. I, however, like backpacks and have been using the Koki Vespa for the last few years. Here's a link to a thread I started when I first got it:
I am partial to my Mission Workshop. Waterproof, very lightweight, quality workmanship, made in USA and lifetime warranty. I have the VX Sanction. Fits my needs in size for my 33 mi RT commute. Only cons are it is a bit pricey, and doesn't have a loop for a taillight.
I bet you also like to play video games while on your bike. Any interest in hopping back in, in say... oh, two weeks?
lol - I kinda miss that game but I haven't really had the time, I've been training for racing lately and don't commute to work anymore so I never pass through downtown. Trying to upgrade to cat 3 at the moment.