There are many different variations of core exercise that you can do. While all will be beneficial in some regard, others will be a notch above the rest.
One of the most important things you want to look for when choosing a core exercise is that it utilizes as many muscle fibres as possible. Usually the types of movements that do this will be ones where your base of support is very small. This would include any movements performed on an exercise ball, done while balancing on one foot (or alternatively, if you are really good, a balance beam) or else done in the air (hanging from a bar for example).
The reason these core exercise movements are so great is because not only will you be working the superficial rectus abdominis muscle (the one you 'see' as being the six-pack), but you will also work the deep muscles that are located underneath this and are also closer to your back.
Additionally, whenever you can utilize a greater range of motion, your chances of seeing more development will be higher. To implement this principle, if you were to perform some abdominal sit-ups on the ball for example, rather than just going back halfway, go back the full distance so that your stomach is very extended. Then when you curl up really think about pulling up from the muscles in the stomach rather than using your upper body to help propel you to be sure you are focusing in on your target. Breathe out while you crunch up, hold for a second while you feel the muscles contract and then slowly inhale while you lower back into the fully extended position once again.
By performing all your abdominal movements like this you should see a noticeable improvement without even having to change the type of workout you do. Abs are one muscle group where it is of utmost importance that you concentrate on what you are doing and not just 'go through the movement'.