The best ab workout is not about doing a lot of crunches. You cannot get ripped abs or a flat stomach just by doing a bunch of crunches. You must have an entire health and fitness program or lifestyle to get the most out of your ab workout.
If you want the best ab workout possible, you need to have the right mindset. You must know what your outcome is and this will be different for each person. Some people want a flat stomach so they look good in a bikini. Some people want to have ripped abs so they can show off a bulging six pack. Other people just want to strengthen their core so they can feel better and eliminate pain.