The best foods for backpacking are actually peanut butter, salami (depending on the brand), cheese, and nutella. They all have roughly the same calorie density at about 70% fat. Toblerone actually comes surprisingly close (even compared to other chocolates/candies).

Raman noodles or tortillas or something like that are generally used as a carrier for those foods to keep you sane, but they aren't very good from a weight to calories standpoint. M&Ms are also common, as they are easy to eat and keep well.

Some people actually just drink cooking oil, but it's rarely worth the tiny bit of extra weight efficiency.