What your favourite ab exercise?

I have always done anywhere from 100-500 crunches a night. These days I do 50 slow sit ups on an exercise ball then 100 fast crunches on the ground. With the ball I sit on it, balance, and lean completely back. Then I sit up very slowly, keeping my arms stretched out in front of me like I'm flying. Or I will let them hang down the side of the ball. Having to balance while doing this really seems to strengthen my core. I have to do these 50 every other day, as my tummy sides hurt the day after.

I really like doing planks.

Also, there is a move on the ball, where you basically are in push up position with the ball on your shins. Then you pull your legs toward your chest (rolling the ball inward) and then extend back. Don't know if this is the best description of the move or not, but the first few times I did this, it felt like I was hardly doing anything... until the next day when I could definitely feel it in my abs.

Pilates. Because it works everything, and while it works everything it works the abs.

YES! Pilates is awesome!

I have always loved the Ab Ripper X within the P90x program. It is awesome in conjunction with using the Ab Doer Xtreme

I believe the best abb specific exercises are when you can incorporate a twisting motion, and, when you move your legs instead of your upper body.
I also think 2 or 3 different ones at lower reps is better than grinding out 100's of the same. Your form can deteriorate real fast and you don't get the same benefit. Always pull in your stomach and engage those abbs, and breath.
e.g.
- Lie on back, and curl/roll lower body up so that your knees come to chest, then go back to start. You can either start in a straight leg position or with knees bent. Add some weight and different 'forces" by anchoring and holding a med ball between you knees.
- Hip raises with a twist, lie on back with legs straight up in air, now lift hips off ground with abbs, and twist one side at the top.
- Alternating leg lifts, lie on back and "scissor kick" to your full extension in both directions, keep legs straight as possible.
- Lie on back, Put a ball (exercicse or med) between feet, and "windshield wipe" the ball from side to side. Keep legs at 90 degree to body, move ball from ground on one side, straight up over your hips to the ground on other side.

And I always like to do transfer type exercises, like :
- lie on ground with feet in air and legs bent 90 degress, now, hold a med ball in hands above head and sit up. Put ball onto your shins. Then lie back down with ball/hands above head. You can add a crazy level of difficulty to this by leaving the ball on your shins each rep, situp, put ball on shins, then pick it back up again on next rep. I wouldn't advise starting with that move.
- Lie on ground in a full straight line from feet to hands with an exercise or med ball between feet, now simultaneously lift hands and feet up in air, and transfer the ball from feet to hands. Now, lie back down and repeat, this time putting ball back inbetween feet.

For crunches, where you situp with your upper body, I much prefer to do these while lying on an exercise ball ensuring lower body stays stable. I find I get a better burn and my range of motion is longer.

There's lots of different variations too.
Another favorite is where you lie on your back, and hold either dumbell or barbell with arms in a bench press type position (the up) and then windshield wipe you legs from side to side. Keep abbs engaged with straight legs and arms. K i l l e r !