Cable crunches .. bar none. There is no better exercise to shock the ab muscles.
Like any other muscle group, if you want it to grow, you need to train for growth. That means heavy, intense and low reps. Forget the 30 rep crunch sets. Do strict heavy cable crunchs with good form, 4 sets, 10 reps HARD, 3 times a week. And most importantly, squeeze (or flex) the living shit out of the abs as you crunch and hold each rep for 2 seconds.
I guarantee you, this will work.
Bula