I think ab training is trial and error for everyone at first, to determine what works for you.
I do core training 3x a week. Breaking them into 3 core sessions...3 sets X 20-25
Session 1 - Cable Crunches to the Front, Cable Crunches to the side, Decline Weighted Crunches, Weighted Obliques Twists, weighted back extensions.
Session 2 - Bicycles, Ball crunches, Oblique Twists, Planks and Side Planks.
Session 3 - Bicycles, Reverse crunches, Hanging Leg Raises, Cable Crunches, Decline Twist Crunches and Planks, and ball back extensions.