Having just a little experience with backpack friendly guns my vote would have to go to the Brocock Ranger XR. These guns are proven tanks and can handle just about any worst case scenario.
I second the Atomic/Ranger XR, am working on getting a folding stock for my Atomic. Want to put it in a small pack and go hiking with my pup Riley. 30+ shots on a fill is plenty for an overnight outing.
I use the airforce talon p. Way more power than let's say a benjamin marauder pistol with a folding ar15 stock and more importantly lighter..... Much lighter. Now the marauder pistol in its stock form is light but I just don't think removing and reattaching the carbine stock is a good idea. Seems like it just keeps getting more and more loose as you do that and I prefer to put my gun in the backpack until I get to my camp or hunting spot. Those other guns mentioned are fine guns but maybe too fine. I like a cheap gun that won't make me cry when I scratch it or it starts wearing from rubbing in the pack. I would like to add that the prod in pistol form with a holosun red dot is light handy and perfect for some applications. I generally like a butt stock but have gone out on ultralight hunts and been fairly successful. It really limits the range I engage at but it's fun and effective.
I would mention the Evol Mini only if a folding mechanism is added. Honorable mentions also go to the Leshiy Classic and Leshiy 2 both with 250mm barrels.
My vote is for the Prod Bullpup. Its the poor mans Layla. Its only 21" long without the LDC. Its all metal so its rugged and durable. You wont cry much if you drop it.
I had an FX folding dreamtact as my backpack rifle. Great gun. I ended up saving and getting the Leshiy 2 in 25cal 350mm. I compared them over several weeks and ended up selling the Dreamtact. I will keep this leshiy 2 for as long as possible. At the end of the day its a tool, an expensive one but even my expensive tools get scratched and dinged so no worries in that department.