What do you consider the best sugar free snack out there that will make you feel full until your next meal?

Oatmeal w no sugar. Slow cook oatmeal for a filling snack with soluble fiber that can help to reduce your LDL cholesterol.
Fiber. Any snack that has a good amount of fiber - say four or five grams - will keep you full until your best meal, and they're generally low in sugar. There are plenty of high fiber, low calorie options out there these days.