To get washboard stomach is not about crunches or a certain exercise, it's fat loss. You can do crunches until the cows come home and not see a thing.

To rev the metabolism and burn tons of calories the best are multijoint movements, ie. the more muscle used over a period of time will burn more than say lifting 20lbs until you can't lift it anymore. That is why I like crossfit. I gives you the routine and challenges you by timing it, so you are taxing yourself when you do it. I literally have an exercise day where the workout lasts 6 minutes and I'm exhausted.