Abs may look awesome but to have a strong core is a lot more important for function. Abs are actually built in the kitchen with strict and proper nutrition, as well as with exercise, but the core is essential for sports performance, strength, posture and balance. The core is covered by movements that involve balancing one or two limbs in extended or contracted positions and can be done stranding, lying, twisting and planking. Oblique work is phenomenal for golf, for martial arts, MMA, boxing and wrestling and pretty much any sport or dance as it involves twisting the lower spine while holding a strong posture.
Planks and side planks (when done properly) are fantastic posture drills and great for engaging the whole body. To work on the lower back and glutes, which hold us upright and affect posture and the appearance of your waist and abs, a couple of great exercises are the hyperextensions and back extensions.
The lower abs are developed through balance work, lifting the knees up high and hanging leg raises. These exercises are fantastic for sprinting, and for kicking, so awesome for performance and overall well-being.
Personally I believe the core should get worked 3 x a week in order to get an awesome shape, as well as awesome functionality and well-being.