What is the best camera backpack for a digital SLR camera?

Best is such a relative term. Do you mean best structural protection & padding / best configurability / easyest access / best value for a given budget / ...
And how much gear do you want to pack?
As for brands, I'm really fond of Crumpler. I use their "Ben's Pizza XXL" shoulderbag when I travel with a dSLR + 3 lens combo (or 1 lens + a bottle of water and a travel guide). Their bags are rugged, well thought out, and what's also important to me: they don't look like camera bags. They have quite a range of camera bags, from small pouches to full-blown backpacks.

I use a Tamrac Adventure 7. I carry My Canon Rebel XT, and 6 lenses, and a canon G5, with more room for extra memory cards, lense filters, chargers, and extra batteries. They also have an Adventure 6, but it is hard to fit a 100-300 attached to your camera in the avalible space. Go to http://www.tamrac.com
to view the various styles. I buy my Photographing equip. at
http://www.bhphoto.com You can get the Adv 7 for $69.00, and the Adv.6 for $59.00. Hope this helps.

Lowepro

Check out the lowepro backpacks. Great quality and alot of choices. I own the Orion Treckker II which is a great weekenders camera bag. I put my Nikon D70s in the bag. It fits the body and lens in the bottom padded portion. The top part has room for my lunch and some additional gear (e.g. 1 more lens).

If you have a lot of gear (many lens) then this is not the bag for you, but Lowepro has several bags that will fit the bill. Check out this link:

http://www.lowepro.com/Products/Backpack...

LowePro MiniTrekker.