This one may have already been mentioned, but Peanut Buttter
seems like a good choice. Very nutrient dense. Lots of calories, fat, protein,
carbs, and it's hecka tasty, and goes well with lots of stuff, or just by itself.
In terms of grab-and-go survival food, I have a little tip for you:
1- Jug of good Whey protein that contains a good mix of Branch Chain Amino Acids
2- Jug of dietary fiber supplement (like metamucil, or generics)
Sure, it's not the most filling combo in the world. But if you get good stuff, that is a sizable chunk of nutrients, which can make up the better
part of quite a number of meals, and which is also very light and can fit in a small space. The items on the list above, alone, almost make up a
complete meal with pretty decent nutrition-- complete with vitamins, minerals, and fiber. What that does not include a lot of is fat and
carbohydrates (as most whey protein only has small amounts of both), both of which are necessary for good nutrition. So you'd need something
additional to take care of that. But 30-60 or so meals worth of protein, fiber, and micronutrients that weighs only a few pounds and can fit in a
small backpack ain't bad.
Also, of course, both the whey protein and fiber being in powder form, you will need access to drinkable water-- but you would need that anyway.
PS-- If you buy the right stuff, this can also be cheap
survival food, which can be stored for decent periods of time as long as they're
unopened, and even a good while after they are.