When trying to achieve the illusive six pack, it' easy to get confused by all the "best ab workouts" available. You'll likely notice that there a multitude of ab workouts to choose one, so deciphering which ones will work the best can be tricky.
The thing to remember is that whenever you are trying to work your core, everything works, for a certain period of time. Everything from high reps, low reps, weighted work, bodyweight work, stability ball exercises, and BOSU ball exercises - it all works until your body adapts and says, "This is easy, I'm not going to change unless you give me a new reason to adapt."
Your ab training should be progressed from stable floor work with your body weight to a unstable surface with weighted work. Progressing to movements that will reduce your base of support as much as possible are the best ab workouts because they will call into play all the muscles within the core, especially those really deep that stable floor work can not target.
Such examples of floor ab exercises that would do this would be crunches, sit ups and leg raises. Eventually you want to do these same exercises against gravity, then a load and then on unstable surface like a stability ball. Body weight exercises like "front planks" and "side planks" should be the foundation of a core program and progressed to one arm planks in the frontal and side planes and then eventually on a stability ball or BOSU ball for further recruitment.
The best ab workouts to make your abs "pop" would be a variety of weighted movements. I'm sure you've skinny guys with a flat stomach but no "eye popping" abs, which is because they have not developed the actual abdominal muscle, just like every other body part. Part of getting nicely chiseled abs is going to be developing the muscles underneath the fat, and weighted ab crunches with cables or on a stability ball with a heavy dumbbell will do this best.
During all your ab workouts be sure to get a full stretch during the eccentric phase of your ab exercises. Going through an entire range of motion on the way up (if doing a crunch on the floor or ball) is not necessary, however, you should be "crunching" your upper abs onto your lower abs during each rep to maximize the burn, recruitment and full development.
The one weighted exercise you will not find in my programs is weighted side bends, as this could make you look wider in the waste, distracting from creating a lean image.
One concerning question most people have is regarding lower ab workouts, typically the most troubling spot. Yes, hip flexion (emphasis more so on lower abs) is important to perform and should be done first in the workout but the reality of the matter is that you cannot specifically separate your abs into upper and lower components. Whenever you perform any type of ab movement you should automatically be focusing on working both the upper and lower portion at the same time.
Good exercise to definitely direct a little more force into the lower area would be the progression of lying leg raises, lying leg raises on a incline and eventually hanging leg raises fully vertical. Lying leg raises on a stability pull is also an extremely challenging and advanced exercise to create razor sharp abdominals.
Finally, the last thing to consider is when you are going to perform your ab workout is training frequency. Again, generally speaking, the more the better assuming your abdominals have recovered. Whether you want to perform it at the end of your workout, before your workout, during your workout or on a day of it's own is up to you. My ab workouts incorporate all methods depending on if you are a beginner, intermediate or advanced. Some theories of that warn you not to start with abs is because abs work as a synergistic muscle for so many of the other exercises you'll do, if you pre-fatigue them before the start of the workout, you may not progress as much during the rest of your workout. I don't agree with this and if you're abs are your weakest link then they should be given first priority when you are the most fresh, the start of your workout.
Incorporating a specific ab workout is only a small part of the battle - diet, lifestyle and overall caloric expenditure through weight training and cardio are the true building blocks to creating a sexy and desired mid section.