If you're looking for flatter abs The Plank is absolutely the best exercise for you. To perform a plank assume push-up position with your elbows bent, forearms extended on the floor. Your body should form a straight line and you should be contracting your ab muscles to stabilize your core. Hold for as long as you can, try for 30 seconds and work your way up to 2 minutes or more.
(Taking pictures is a good way to fix your form too, in this pic I realized my elbows should be slightly more aligned with my shoulders.)
Demonstration video below: