Conservative Scalawag has an [interesting post about making your own MREs]*. I’m not a huge canned tuna fan, but it can’t be any worse than Country Captain “chicken” (it might be chicken, the jury is still out). Personally, I always like the beef enchilada.
But I think his choices here are great! I’d estimate the calories in his packet to be close to 800 calories, and probably pretty good on weight too. For backpackers, weight is a pretty big issue, so you tend to favor foods that are really calorie dense in terms of weight. Nuts are a great choice, so chocolate coated nuts are even better. One other thing to consider is the lowly pop tart. While not entirely durable, the 8oz package packs 400 calories of yummy frosted goodness.
* Dead link removed