My classic answer is banana. It's a wonderous fruit and can spice up any number of foods, medicines, candys, etc.
But, when it comes down to it if I can have just one fruit to eat nowadays I'll pick watermelon. The groceries now have seedless easily available and the stuff I've been getting lately is so sweet that it makes me weep out of pure joy. It's truly better than candy and I like to eat bowl after bowl of chunked watermelon, until I feel sick to my stomach.