Crunch is the best exercise for stomach. Crunch is easy and you don't need any equipment for basic crunch. But I hope you have already found that just doing abs crunch will not ensure a flat stomach. Spot reducing will not work unless you have already reduce fat all over the body. Body can not be forced to lose fat by working on the muscles on that area only. So if you are really over weight and excessive fat around the waist line, don't spend time now on abs crunch. Rather slow distance walk will be a good alternative for you, while your primary goal should be to concentrate on taking fewer calories of food daily.
But if you have already reduced fats all over the body and now want to concentrate on flattening your stomach, then abs crunch will be very effective for you.
The basic form of crunch is lying face up on the floor with knees bent. Then the shoulders will be raised and curled towards the pelvis. Here main stress will be on the abdomen area. Keeping the basic same, i.e., stress on stomach, different crunches are used by people of different strength. For some people basic crunch is too easy, and also if someone is specifically looking for six packs, more difficult crunches are performed. Let me discuss few other abs crunch exercises.
Reverse Crunch : Here leg is raised rather than chest. Reverse crunch is a bit more difficult than regular crunch. Twisting Crunch : Here one shoulder at a time is lifted and body will be twisted at each crunch. Unlike regular or reverse crunch here major emphasis is on the oblique.
Bicycle crunch :
The bicycle crunch is very effective for toning both rectus abdomimis and oblique. Here alternating elbows and knees are moved together. Motion of the leg would be similar to that of legs while pedaling a bicycle.
Which crunch you should choose ?
As you see there are many types, the question is which one you should choose or should you do all the ones.
Each crunch has different objective, like Bicycle crunch is more effective for rectus abdominis and oblique, where as twisting crunch is helpful for oblique. If you have already achieve a good shape around your stomach, you should do alternatively all the ones. But if you are just a beginner, go for regular crunch. After some practice, regular crunch will be easy, then you can switch to difficult ones.
How many repetitions are necessary !
Don't try to achieve your goal too early. If you overdo, after certain time you will lose the enthusiasm. Beginners should be happy to do two sets of 10/12 repetitions. Afterwards when things will become easier, you can do two sets of 20/25 repetitions.