Dead Bugs. I always tell my clients, "If this is easy, you're not doing it correctly". This is one of the few exercises that challenges the rectus abdominis, obliques and transverse abdominis all at once. It requires, balance, coordination and ab strength.
How to: Lie on the floor (supine) with your arms reaching up toward the ceiling. Lift both feet off the ground so your legs are bent at 90 degrees. Press your low back against the floor and keep it pressed there the entire range of motion.
Slowly, extend your right arm over your head and parallel to the floor and lower your left leg toward the ground at the same time, keeping that 90 degree angle. Tap your heel to the ground. Exhale as you return to the starting position. Repeat with the left arm and right leg.
To make this more challenging, when lowering your legs, straighten them more. This creates a longer lever and therefore, a more difficult exercise.