The highest production tree is actually an invasive in my region, the Autumn Olive. Just North of me in Southern Illinois they populate into the hundreds of thousands of trees. They produce gallons and gallons of delicious fruit. They also fix nitrogen. So, they make excellent nurse trees.
I would also recommend pears and walnuts as they produce copious amounts of fruits and nuts. The highest production shrub/cane I would recommend blackberries/raspberries depending on where you live. The warmer it is the better blackberries do, and vice versa. Blueberries also have a really nice neglect factor like the rest of what I mentioned. As long as the soil is acidic (4.5 to 5.5), they'll grow in nutritionally poor soils.
As far as apples go, it just depends if you have issues with coddling moth. If you don't then you can plant cultivars like Liberty, Macfree, Freedom, and William's Pride. We grow Liberty and have no disease issues whatsoever, but coddling moth is devastating here. So, we have to micromanage it unfortunately. Same with plums and peaches in our area with plum curculio.