That is a real difficult question as I love most summer and stone fruits.
Once upon a time I would have said strawberries however the last few years the strawberries taste of little, and I have virtually stopped buying them, as they mostly do not last long either.
Perhaps I would say water melon - it is crispy, sweet and doubles for me instead of a salad with some meals. It is low calorie and refreshing and is therefore one of the more versatile fruits.