At first glance, the best and most effective six-pack ab exercise won´t be recognized as abdominal-specific, because it isn´t the classic and very popular exercise like crunches, leg raises or sit-ups that I´m talking about. Obviously, even crunches don´t offer a really functional core training, hence there are much smarter ways to train and shape the center of your body.

Six-pack Abs Requirements

To get six-pack abs, you have to meet two essential requirements:

Muscles	Strong core muscles are the basis, not just for good-looking abs, that is nothing more than a pleasant side-effect, but for a pain-free back, a good posture and an outstanding training performance, which requires the core muscles to be as strong as possible.
Body Fat	Men need to have a body fat percentage of 10% or lower to show a six-pack ab. Otherwise they remain hidden below a layer of body fat. That, however, means no functional disadvantages at all, you simply don´t have visible abs.

Therefore you have to keep your body fat as low and core muscles as strong as possible.

Complex Training vs. Isolation Training

Currently, the most popular ab exercises are isolation exercise that involve certain rolling-in movements of the upper body, which is no functional way to train your abs. The main purpose of all core muscles is to stabilize the trunk against external forces. Beside stabilization, the obliques serve as torso rotator. Let me summarize:

Isolated core training through upper body roll-ins or side-bends is a meaningful act, but does not provide a functional and natural way of core strengthening.

Also the percentage of body fat, as the second important pillar for a six-pack abs, can be influenced more effectively by doing functional and complex exercises, since you activate a large number of muscles simultaneously. This leads to an increased hormone secretion and increased fat burning. A reduction in body fat requires a certain amount of active muscle and an adequate diet, which leads to insight number two:

The more muscles you activate simultaneously during an exercise and the more muscle mass you own, the higher your potential for burning body fat. Therefore you have to do complex exercises, requiring a signficant and functional participation of your core muscles.

The List: The Best Six-pack Ab Exercises

I will divide this list into two categories:

Ab Exercises whose target muscles are mainly abs.
Non-Ab Exercises that activate a large number of muscles and put high demand on your core muscles.

Ab Exercises

Exercise Name	Image and Link	Comments
Crunches, Seated		It is a very effective crunch variation. You can easily adjust the training resistance to even hard levels. The advantage over crunches is the nonexistent external pressure on the vertebral bodies.
Plank		Plank is an easy-to-do stabilizing exercise. Beware to keep your abs tensed and to avoid any hyper-extension of the lower back.
Side Plank		Side planks train you obliques. It relates to planks and involves many other muscles to keep up the position.
Russian Twist, Seated		It is a great exercise for obliques while abdominals serves as stabilizer. Remember to avoid any lower back hyper-extension throughout the motion.
Mountain Climbers, Floor Sprint		Das eigentlich Anziehen der Beine wird allein von der Hüftbeugemuskulatur realisiert, die einseitigen Lastwechsel und das Beugen des Oberkörpers aktivieren jedoch sehr stark die schrägen und geraden Bauchmuskeln. Die Liegestützposition erfordert eine Anspannung beinahe aller Muskeln der Körpervorderseite.
Reverse Crunch		The upper back fixation combined with the hip pull up results in a significant abdominal workout while relieving the lower back.

Non-Ab Exercises

Exercise Name	Image and Link	Comments
Front Squat		Front Squats are a fantastic leg workout that requires your torso to be kept as upright as possible, which on the other hand implies high stabilizing effort of your abdominal muscles. The large number of participating muscles makes front squats one of the most effective exercises ever.
Suitcase Squat		It is a squat variation that, depending on the level of resistance, will train your complete muscles, especially obliques.
Overhead Squat		Holding the barbell above your head is a real challenge for your upper body muscles, while the lifted center of gravity put more emphasis on stabilizing the trunk. Like front squats, it is one of the most effective exercises.
Renegade Row		Balancing the side-to-side load changes really train all core muscles, especially obliques. At the same time the dumbbell pull-up movement and push-up position hit all back, chest and arm muscles, which makes it an extremely demanding exercise.
Barbell Deadlift		Deadlift, as one of the classic power lifting and strength training exercises, focuses on training your thighs, lower back and traps. Thus, it is one the best six-pack ab exercises, because he use of relatively high workload enforces a very complex muscular activity