Lots of people have posted answers saying what they like to eat. However, the OP asked a very specific question, which was: "What is the most efficient food to take a for a 12-15 day hiking trip? [...] Assume I have no taste at all and don't care about eating the same tasteless thing every day if necessary." She specifically stated that her only criterion was efficiency.

By this criterion, there is one very well-defined answer to her question, which is that she should bring food that is purely composed of fat. Fat has an energy density of 9 calories per gram. This is a pretty good approximation for all fats. If you look around for pure-fat foods with very little water in them, basically all you find is cooking oils such as olive oil. For comparison, granola is about 3 cal/g, oatmeal 2 cal/g, cookies 6 cal/g. So by the stated criteria, the OP should put a large jug of olive oil in her pack.

If you use calories per liter as your figure of merit, you get about the same answer: cooking oil. This is because oil is about twice as dense as most foods that you'd actually want to eat.

Do I recommend a diet of pure olive oil for a 2-week backpacking trip? Of course not, but that just shows that the OP needs to state more realistic criteria. Not only would any human be miserable on this diet, but carbs serve as the primer for fat metabolism. The human body can't digest fat without having carbs as well. Without any carbs, you get ketosis. See, e.g., McArdle, Exercise physiology: energy, nutrition, and human performance, 1986.

A good starting point in setting more realistic criteria would be to decide on some balance of fats, carbs, and protein. A standard recommendation is about 50% of calories from carbohydrates, 35% from fat, 15% from protein.

Lots of people have posted answers saying what they like to eat. However, the OP asked a very specific question, which was: "What is the most efficient food to take a for a 12-15 day hiking trip? [...] Assume I have no taste at all and don't care about eating the same tasteless thing every day if necessary." She specifically stated that her only criterion was efficiency.

By this criterion, there is one very well-defined answer to her question, which is that she should bring food that is purely composed of fat. Fat has an energy density of 9 kcal (37 kJ) per gram. This is a pretty good approximation for all fats. If you look around for pure-fat foods with very little water in them, basically all you find is cooking oils such as olive oil. For comparison, granola is about 3 kcal/g (12.5 kJ/g), oatmeal 2 kcal/g (8.4 kJ/g), cookies 6 kcal/g (25 kJ/g). So by the stated criteria, the OP should put a large jug of olive oil in her pack.

If you use calories per liter as your figure of merit, you get about the same answer: cooking oil. This is because oil is about twice as dense as most foods that you'd actually want to eat.

Do I recommend a diet of pure olive oil for a 2-week backpacking trip? Of course not, but that just shows that the OP needs to state more realistic criteria. Not only would any human be miserable on this diet, but carbs serve as the primer for fat metabolism. The human body can't digest fat without having carbs as well. Without any carbs, you get ketosis. See, e.g., McArdle, Exercise physiology: energy, nutrition, and human performance, 1986.

A good starting point in setting more realistic criteria would be to decide on some balance of fats, carbs, and protein. A standard recommendation is about 50% of calories from carbohydrates, 35% from fat, 15% from protein.