The Bicycle Crunch is the best ab exercised according to research done to uncover the best exercises you can do to activate the rectus abdominus.
A study done at San Diego State University compared thirteen common abdominal exercises in an effort to determine the best. Each exercises was ranked for muscle stimulation–measured with EMG–in the rectus abdominis, the external obliques and internal obliques.
Another great thing about this work out, you do not need any equipment.
Do the Bicycle Crunch:
Lie flat on the floor with your lower back pressed to the ground and contract your core muscles.
With your hands gently holding your head, lift your knees to about a 45-degree angle.
Slowly, at first, go through a bicycle pedal motion.
Alternately touching your elbows to the opposite knees as you twist back and forth. Continue to move continuously, as if pedaling a bicycle.
Breath evenly throughout the exercise.