Hanging leg raises, weighted and non weighted crunches, reverse crunches, weighted and non weighted planks, anything that contracts and relaxes the ab region like any other body part will strengthen the abs. But the most important exercise for developing sharply defined abs is....EXERCISING DISCIPLINE IN DIET...10,000 crunches won't get rid of 10 lbs of belly fat and a poor diet..Everybody has a six pack whether they workout or not. We just cover it up over the years and think it's gone. Not gone, buried...Got to dig it out, by putting the pudding spoon down..lol