The bicycle was ranked as the best ab exercise by the Amercian Council on Exercise because it uses every muscle in your abs to create a strong, lean torso. Make sure you pull your abs in (draw your navel towards your spine) and keep them contracted throughout the movement to protect your back. Don't hold your breath and go slowly so you can really feel it! Do 10-16 reps.
The plank is another great exercise that challenges your Transverse Abdominis as well as the muscles of your back. You can do this move on your elbows or on your hands. Position yourself as though you're about to do a pushup and hold that pose, making sure your body is in one long, straight line from head to toes. Don't let your belly sag and keep your neck straight. Hold as long as you can!
Unless you live under a rock, you probably know that using an exercise ball provides for a very challenging ab workout. Instead of doing crunches on the floor, lie on an exercise ball. You'll challenge your abs in a whole new way while also working other muscles in your body like your legs and butt. Make sure you don't pull on your neck and roll forward a bit if the move is too difficult.
The reverse crunch is great for working your obliques and it helps you target the lower portion of your abs as well. There are many ways to do reverse crunches, but one of the best is to put an exercise ball under your knees. Make sure you contract your abs and use them to lift your hips off the floor. It's a very small movement and you shouldn't swing or use momentum.
This exercise also challenges the abs in a different way, but it's an advanced exercise so pay close attention to your back. The movement involves rolling or sliding your hands out, using your abs and back to keep your body straight and then contracting them to pull back in. You can use anything from a Torso Track or Ab Wheel to a couple of pieces of cardboard for this movement.