Recently read an article that said weighted crunches (lay on the floor, get the rope as close to your head as possible, and put the weight high enough so you can only do 6-8) is the best bet.
They were going on the principal that doing hundreds of crunches might make for better endurance but really doesn' t overload the muscles, which makes them bigger.
You don' t bench 135 x 25 times and then just keep increasing.. Most people in the forum will tell you to train heavy. So why any different with abs?
Granted, each individual will respond differently. That goes back to the calf debate if you should go heavy for 6-8 or 15-20. Some people get stronger at the 6-8 range but notice no size increases.
So this is just a general statement above.
And of course, diet affects how visible the abs are since males seem to carry most of their weight around the mid-section.
So my vote is for weighted crunches, then incline crunches (holding a weight to the chest). Of course incline weighted crunches, you might be able to hold more weight then is safe.