Do them all at once. Decline sit ups works my abs and obliques pretty hard. When you can get 15-20 reps, hold a ten pound plate behind your head. Progress from there.
I would also do hanging leg raises. Don't hang from those stupid looking slings. Hang from the bar with your hands. Side effects are improved grip and upper back strength.
You could include a rotational exercise (wood choppers, Russian twists, full contact twists) or side bends too.
Sit ups, hanging raises, and a rotational exercise makes for a well rounded routine. Don't forget your low back