Okay, there I said it! There is actually an ab exercise that I like and it's the Plank. Now I'm not a huge fan of practicing the Plank (I'm easily bored) but I do like the RKC Plank (do a quick search on Google) and practice it. Why? Because the Plank is what you are using when you are doing Push-ups, Deadlifts, Farmer's Walk and Carries, and pretty much any overhead technique (even Dead-hang Pull-ups) that require you to brace and hold a locked position.
For more on how to use and train the Plank check out this article by my friend Andrew Read.