Before I give you the top lower abs exercises I want to make sure that you know that these exercises alone will probably not give you the results that you want.  They will definitely strengthen the lower abs but in order for the lower abs to be seen, the following must be included in your plan:

• Cardio – intervals are the best
• Strength Training – Must include all major muscle groups
• Diet – A calorie deficient diet
• Other Core exercises – You need all abs muscles including lower back.

The 3 best lower abs exercises

1. Reverse Crunch – This is best performed on a decline bench.  Lie on your back and use your hands to grab where normally you would lock your feet in.  Start with straight out at so they are parallel with the floor.  Pull your knees to your chest and lift your butt and lower back off the bench.  Hold for 1 to seconds and slowly return to the starting position.
2. Hanging Leg Raises – This can either be done hanging from the pull-up bar or using the Roman chair.  Start with your legs straight down lift your legs so your feet are at eye level.  Try not to bend at the knees.  Hold for 1 to 2 seconds and slowly lower your legs.  Stop before your legs are straight down so your abs stay fully engaged.  This is difficult exercise and you may need to start by just bringing your knees up.
3. Abs Bicycling Modified – This can either be done on a foam roll or a bench.  Scoot your butt to either the end of the bench or foam roll.  Either grab onto the bench by your hips or place your hands on the floor for balance.  Come up to a crunch position and hold.  Pull one knee to your chest.  The other leg is going to be straight and actually lower than the rest of your body.  That why you need to either be on a bench or form roll.  While maintaining the crunch position slowly pull the straight leg to your chest and straighten the other leg to slightly fall below the line of your body.

All three exercises will rapidly strengthen your lower abs.  Start out slow.  Just by doing 10 you will it the next day.

Check out these Lower Ab Exercises
Lower Ab Exercises Part 1
Lower Ab Exercises Part 2