BEST ABDOMENAL EXERCISES

The best abdominal exercises are the ones which target specific areas on your abdomen.
You can and should be very specific when doing abdominal exercises in terms of region of your abs being worked.
If you had a six pack and wanted to work your lower abdominal region you would perform the exercises which involve bringing your legs up in the air.
Your legs must be straight.  Most people perform this exercise improperly as they bend their legs when raising them. 
Straighten your legs and slowly lower them almost to the ground.
Raise them in a slow and methodical manner all the way up until the are straight up in the air.
This is one full and complete repetition.
Perform this ten times for a complete set.
Do them slowly as this particular lower area of your abdominal wall has a very thin muscle mass and is susceptible to injury, strain or stress.
If you are interested in working your upper abs you would want to be flat on your back with your legs bent.
The more severe bend in your legs works your extreme upper portion of your abs.  Slightly bending your legs will emphasize working the middle two of you six pack.
This is how to properly work each individual area of your abdominal wall.