Granny smith -- green, hard, break your teeth before they are ready, but when they're ripe they're pretty much the best of green apples. Excellent cooking apple.
1 year old bare root seedlings grown from Granny Smith apples purchased at the organic food store.
Photo Credit: Kolforn (Wikimedia), CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons