To me, Straight Leg Hanging Hip Flexion (AKA Hanging Leg Raises) is the Daddy of Ab exercises and best demonstrated by Gymnasts. Performed with good-form (controlled, no swinging), many top physique athletes will struggle to reach 30 repitions. Think of this when you're performing 300 Sit-Ups!
To lesson difficulty, simply perform as Hanging Knee Raises, or Lying Hip Flexion.
For ideas on structuring your workout to best include Abs, check out our 'Workouts' section.
I'm sure that many of you will also find the following Gym Professor article of use 'How to get Six-Pack Abs'.