Its generally acknowledged that a healthy, low-body fat percentage is an important for showing off abs, however while you can lower your levels of body fat, you can equally increase your muscle mass.
While spot reducing as been shown to be a myth, surely altering the localised ratio of muscle to fat will show some improvement. Consider, if you had fixed volume of fat around your arm, if your biceps and triceps grew, while this volume remained the same, the thickness of the fat around the arm should surely drop, improving the visibility of the muscles. Surely, it would be the same for your abs.
Additionally, while the core make be worked during other activities, such as squats and deadlifts, the extent to which the abs will be to support these activities. Again, while deadlift engage the biceps when holding the bar, I doubt people would recommend deadlifts to improve bicep size or shape.
Lastly, Dr. Stuart McGill has come out with strong evidence that crunches (or activities that involve lumbar flexion) are strongly associated with poor lower back health. However, the advice that situps and crunches (weighted crunches even) builds abs is still passed on.
As such, the question is this:
Assuming one already had their diet in order with a reasonably low body-fat percentage, what exercises and with what set and rep ranges, should one do to safely improve the size and strength of the muscles in the abdominal region, focusing specifically on the abs and obliques?