Unfortunately, there is no one best exercise for training the abs.
Focus on doing the movement right so that the exercise works for you. This means no sloppy, or halfhearted movements.

I just can`t get rid of the layer of fat on my stomach. What should I do?
The best way to burn fat is through diet modification and exercise. Cut out empty calories and eat sensibly. Simple exercises such as walking, swimming, cycling or running are effective for burning calories.

All I want is a toned stomach. So why do I need to strengthen my back?
Always remember to strengthen your back muscles when you do your abdominal training routine. Strong back and abs are necessary to create muscular balance in the body. For example, if you train the abs and forget about the back, you will end up creating weak back muscles.