So, lets examine one of the most common exercises out there: the sit up. Sure, the sit up will highly focus on the ever eluted six pack, but it will also do some other things such as depress your rib cage, which in turn increases thoracic kyphosis while limiting thoracic extension. When this occurs, the scapulae are put at a sub-optimal position which can also induce impingement in the shoulder. Would you rather have JUST good looking abs or good looking abs on top of a rock solid, strong core which will keep away an array of back issues when your older? I think I'll take option two.
Now, what is an exercise which can do all of this? The turkish get up. This movement utilizes every muscle of the core to promote the functions discussed above and will also give the gauchos and senoritas out there something nice to look at. The turkish get up utilizes several different planes of motion and bar none will help improve a long list of things: improved functional movement patterns, core strength and stability, hip extension and mobility, shoulder stability, and thoracic extension. Where can a crunch or sit up give you all of that?
When progressing through this exercise, it essential to....PROGRESS. Dan John is a believer in breaking the movement into several components and progressing someone from body weight to balancing an empty cup on their fist, then balancing a filled cup, and then finally a kettlebell. I find this progression to be optimal as well. So, start small and peep the video below for an explanation via Gray Cook.