Robert: That's a good question, because it forces everyone to think about nutrition and not just what tastes good.
As I'm sure you know, it's best to have a balanced fruit diet instead of focusing on one fruit. No fruit can provide all the nutrition you need. For instance, bananas are high in potassium while oranges are high in vitamin C. Backpackers need both.
However, if I had to pick one fruit it would probably be guava.
Other candidates would be: all berries; kiwi; papaya; avocado. Pomegranate is fashionable now, but it's expensive.
Here is a useful table that includes
calorie density and nutrition of many fruits and vegetables.
When I'm backpacking, I love to eat avocado with tomato, whole wheat bread and (soy) cheese. I'll also carry carrots and whatever fruit I can get my hands on. Eat them early on in your trek to lighten your load quickly.
Although fruits are vital, veggies are even more important. They have more nutrition and less sugar than fruit. That sugar is important while backpacking because it gives you calories. Grapes and dates, for example, are jam packed with calories because they have lots of sugar. And they're better for you than M&Ms.
At home, I would focus on veggies. Eat deep leafy greens, broccoli, spinach, sweet potatoes, red peppers, and carrots. Eat lots of fruit at home too, but remember that veggies provide the best bang for your nutritional buck.
Finally, don't forget to eat something evil every now and then.
