What is the best back pack for the Zombpocalypse?

Macpacmake very good bags. Make sure it's one with their 'Aztec' fabric. The 'Pursuit' and 'Ascent' models are great examples of no-nonsense toughness (make sure to get the 'classic' version of either).

Also, while I've never owned one, I've heard very good things about the bags by Cactus Climbing. Though expensive, I've heard they'll last pretty much forever and take a good beating while at it.

Options like these tend to trade featherlightness for durability, but that's what you want from an apocalypse/emergency bag. They offer less doodahs and watsits for zombies to grab hold of, but still provide places for you to hold water and that all-important axe.

For an actual zombie apocalypse I think that you would want streamlining for reduced purchase by the walking dead, good suspension, and waterproofness. On those counts, the Arc'teryx Arrakis packs (http://www.arcteryx.com/Product.aspx?EN/Mens/Packs/Arrakis-65#Over_60L) would be a good choice. If you are looking for a grab-and-go with lesser capacity but similar properties the Patagonia Stormfront might be a good choice (http://www.patagonia.com/us/product/patagonia-stormfront-waterproof-pack-fly-fishing?p=49152-0-950). Me? I'm working on speed, agility, endurance and being able to outrun you :-) That said, fingers crossed for shamblers rather than sprinters.