I'm typically reluctant to designate a "best" exercise for a given bodypart because nearly any exercise you do will have certain positive effects. But if we're talking abs, I'd have to go with the abdominal crunch for two reasons: 1) You can easily add resistance, and 2) it's safe.
Adding resistance is the key. Because your abs play a dominant role in so many sports and activities, being strong in that area can impact your performance tremendously. Naturally, as a bodybuilder you need to have well-defined and well-developed abs. The definition comes from a strict diet, the development from progressively increased strength.
But strength is also important for other reasons. Can you imagine doing a squat, lunge, deadlift, pressdown, etc. without a strong midsection? Unfortunately, many ab exercises can stress your lower back, especially when you add resistance; situps and leg raises top the list. But not so for crunches. Depending on your sport's strength requirements, you can safely load up with as much weight as you can handle.
Key points in exercise execution include:
1) Lie faceup on the floor and drape your legs over an exercise bench. If possible, select a bench that allows your hips and knees to form 90-degree angles in this position.
2) Fold your arms across your chest or hold a plate on your chest. Avoid placing your hands behind your head; if you do, you can end up cranking your head forward, which could in turn hurt your neck.
3) Take a deep breath and slightly tuck your chin into your chest as you curl your upper body and bring your shoulders and upper back off the ground.
4) Keep your calves on the bench throughout the exercise, and exhale as you reach the last third of the movement.
5) Breathe in as you lower yourself back to the ground under control, then repeat.
Abs: The Truth
Abs and Obliques
In One Workout