This question was probably done before but I thought it'd be fun to do it again.
The backpack should have things you could reasonably have stocked up on at home or could come by easily. (Except for the guns.) They should all be able to be carried in a normal sized backpack.
You can carry one extra weapon in your hand like a shotgun with aiming mechanism too help your accuracy in shooting the zombies.
1. Shotgun with infrared aiming device and extra bullets
2. Handgun and extra bullets
3. 2 Bottles of water
4. Antibiotics like cipro and pain killers
5. Hunting knife
6. Flashlight and extra batteries
7. Nutrition bars
8. Rope with hook tied to it
9. Bandages
10.Packages of seeds to grow some type of vegetables to grow if place found to hold up in
11. Transister emergency radio
12. Two flares and water proof matches
Even with all this I don't think I would last even 5 minutes in a zombie attack. 
( lol
1. keys to the tallest building in the world. ( hopefully the elevators working)
2. shotgun
3. lotsa food and water
4. a lot of batteries
5. parachute
6. medicine
7. medical suplies
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