The primary function of the ab muscle is to shield the organs and prevent bending of the spine. So the best way to train it is to cater to its natural tendency, rather than do sit ups and crunches, do planks and side planks to really hit your obliques and abs. Couple that on certain days with crunches or hanging knee lifts for a solid ab workout. Remember that it is still one muscle with tendons crossing it, not several muscles, so dont overtrain your abs, and dont ignore a part of it, especially the lower abs.
As stated, the squats would be beneficial for endurance, but i personally wouldnt do them everyday in those numbers.
Personally, i would combine 50 pushups, 3 minutes of planks each, and 50 deep squats with a stretching routine to really limber up and utilize the muscle groups.
But thats just me, hope it helps.