Joe Decker was named Worlds Fittest Man by the Guiness Book of World Records. He is an ultra endurance power athlete, completing in events like Badwater (135 mile run in Death Valley) and the Grand Slam of Ultra Running.
He suggests lowering body fat between 5-10% if you want nice "looking" abs. As far as strong, functional abs go, these three exercises are his favorite:
Weighted Sit-up
Your abdominals are just like any other muscle. They really need to be challenged to strengthen and grow. Find something to anchor your feet. I like to use a set of heavy dumbbells or something similar. Next, find a heavy object to place across your chest and do sit-ups while holding it as close to your chin as possible.I try to do a couple sets--performed slowly--of about 25 reps.
Hanging Knee Raise
Find a jungle gym or a pull-up bar. Grab hold with both hands and hang from the bars with your arms straight. Pull your knees up to your chest, pause and slowly let them back down. For best results, go slow and avoid momentum.
Russian Twist
This is the last core exercise to round out my three favorites. I find it to be one of the most old fashioned, but also the most effective. Simply sit on the ground and, again, find something to hook your feet onto to keep you anchored. Lean back until you are in a half-way sit-up position. With your abs tight, rotate from side to side, slowly pausing when you reach the end of each rotation.
