What exercise is the best for this area?

A reverse crunch definitely!! But when you are doing it keep your legs in the air at all times, don't drop them or roll back and forward too quickly as this can just strain your muscles but not actually burn anything!!

I do 500 ab crunches a day 5x a week, the plank, and Zumba just to get rid of my baby pooch...so far so good. I have been at this for 2 months and have lost 2 pant sizes!

For the area under your arm, try one arm rows with slightly heavy weights (about 10-20lbs). Take a dumbbell in one hand. Bend over at the hips, back flat, knees slightly bent. You might wish to support your other hand on a chair. Now, without flaring your elbow out to the side, pull the dumbbell up towards your underarm pit. Keep it very close to the side of you body at all times. Come up quickly then lower slow for a count of 3. Do this 12 times. Rest 20 seconds. Then come up slowly for a count of 3 and go down quickly. Make sure you don't snap your elbow or lose control on the way down. Keep control of the weight at all times. Do this 12 times. Now do a 2 count up and a 2 count down, or singles, 12 times.

Also, if you have a band, you can do straight wide-arm (like the letter Y) overhead extensions. Keep the tension on the band at all times. Grab hold of each end in each hand and hold in front of your chest. Make sure there is no slack in the band. Now lower to your hip area, then raise with straight arms up, in front and over your head. By the time you get to about your shoulders, you'll feel the tension in the band begin to kick in. GOOD! Don't let up. Go straight up overhead then slowly return the band back down to your hip area. Do this 12 times, rest 20-30 seconds and do another 12. Repeat 3 times.

For your lower abs, grab the band again. Lay on your back with knees bent and feet in the air at a 90 degree angle. Place the band across your knees and cross it behind your buttocks. Grab hold of each end tightly. Keeping your head, arms and hands (palm down) on the floor, slowly lower your hips and knees off the floor towards the ceiling. Use a similar count for this exercise as mentioned in the one arm rows. This one will have those abs smoking hot and burning!!