The bicycle crunch is identified here as the best exercise for the abs because it isolates both the retus and the oblique muscles. Independent studies show that electrical activity in these muscles during this exercise is much greater than with traditional crunches or sit-ups. It is simple to do, requiring only an exercise mat and a little time each day.

The bicycle crunch is accomplished by doing the following:

• Lie flat on the floor with your lower back pressed to the ground.
• Put your hands beside your head.
• Bring your knees up to about a 45-degree angle and slowly go through a bicycle pedal motion.
• Touch your left elbow to your right knee, then your right elbow to your left knee.
• Breath evenly throughout the exercise.
• Keep your feet above the knees throughout the exercise. This will help isolate and take strain off your back.
  

Here's a video: