I feel like the greatest of the ab workouts is dancing. You might not think so, but you can do so many crunch-like moves while standing and moving around that can really carve out a six-pack!
I had been doing crunches for years before I tried a dance workout. In just a few months of doing it, I had better sculpted abs than I had gotten from years of crunches!
Also, this type of workout is more fun than boring crunches. You get to move to music while you tone yourself, and anything that is fun is easier to keep doing on a regular basis. It's hard to lose interest in dancing, but it's easy to lose the motivation to do a bunch of crunches.