I always like doing reverse crunches with a light medicine ball between feet on a decline bench. I usually will burn out only after 15 reps. I also like doing the hanging leg raise and wind shield wipers. This is where you hang from a pull bar and bring your feet to eye level keeping your legs straight. You then bring it down to the side and back up around to the other side like the wind shield wipers on your car. Right no I can only do a few. the last one I do is the crunch on the stability ball holding a dumb bell. These 3 exercises makes abs sore for days after doing them.
Happiness lies, first of all, in health - George William Curtis Lower Ab Exercises