Spread Eagle Sit Ups with Med Ball
This one is as old school as you can get. If you think your abs are strong you will find out real quick just how strong you are.
Get on the floor and spread your legs apart (and anchored under something like an EliteFTS power rack). Hold the med ball over your head and pull yourself for a laying position to a sit up position.
Keep your torso as tight as you can.
Three sets to failure is about as much as anyone should do.