I use this Fancier Kingkong 40, as stated earlier. See
Fancier KingKong 40 Pro Camera Bag Backpack
What makes this pack so much better than many of the others is:
1. Mesh back, less sweating on long hikes.
2. Storage for one DSLR with lens attached, and 7-10 additional lenses in the main compartment. Seperate smaller compartment which is good for food, additional clothing etc. Small filter / memory card / cleaning materials compartment. Laptop pocket.
3. Tripod carrying capability.
4. Ability to add additonal side bags.
5. The camera and lens can be removed from the back-pack without you having to take the back-pack off your back completely, you just slip off one arm, sling the bag around to your front/side and remove the camera out of the side access. The bag has side access left and right so suits both left handed and right handed photographers. So, you need not put your bag down to get your camera out.
6. There's a rain cover, which is attached in a bottom compartment, easy and quick to whip out if it starts raining during your walk.
7. The shoulder straps are wide and comfortable and there's a chest strap which is great for longer hikes.
8. I have used this pack on 20km (12 mile) and longer hikes and it's worked great.
9. It's very well padded so your camera is unlikely to get bumped or damaged even if you should drop the bag.
10. The inside is bright orange, so your camear gear is easy to see, even when it's quite dark. All compartments can be sized to your requirements.
The only negatives I can find is that smaller lenses sometimes slip out of their compartments and drop into the lower compartments when you've not sized the compartments tight enough for the lens. That's easy to fix though. And, I would have liked more space and better dividers in the small front compartment for cleaning gear and for filters.
On long hikes, I use the one side access pocket for my water bottles, and that reduces my (addiitonal) lens carrying capacity to 4. There's enough space in the top compartment for small first aid kit, space blanket, food, and wet weather gear - all essentials for a mountain hike.