Is there an ultimate diabetic snack you can eat as much as you want?

None: There is no need for snacks if you have diabetes. You will need more Insulin secretion (or injection) to cover the snack. Don't treat your sugar by food, treat food with medicine. It's also harder to lose weight if you eat snacks, and your body is always in the post-meal state which is much harder for it to control. There is nothing you can eat as much as you want except water and air.