The low rankings for Stud Hi/Lo are exactly the same as those for Omaha Hi/Lo. The best possible (or 'nut') low is the Five-low hand known as the wheel: 5-4-3-2-A. Other lows are calculated using the highest card first, so 7-6-5-4-3 beats 8-4-3-2-A, etc.
For more information on low hand rankings, please refer to the following pages: