As with most "best exercise" questions, the answer depends on what you're trying to achieve. The two main goals people have are flat abs and 6 pack abs. To have a flat abdominal area you need to train the deeper abdominal muscles. This is done by doing core stabilising exercises. The 6 pack muscles are the top layer of ab muscles and are trained by crunches and similar exercises - any exercise in which the upper body and lower body come closer together against a resistance.  If you want to train all your ab muscles, bicycle crunches are a good all-round exercise.