The Problem with Ab Exercises
Most ab workouts/routines you see are mostly if not entirely composed of bodyweight exercises. These are great for beginners, because there is such a sharp learning curve due to the high body weight and lack of muscle. Unfortunately, they have a significant disadvantage.
The problem is as your muscle grows stronger and adapts to the exercise, your intensity decreases because your bodyweight is effectively static. This means that as you progress your only option will be to do more reps at a low intensity, which is more conducive for building endurance.
Use Weighted Movements
Since optimal muscle growth occurs under high intensity with low reps, the most effective ab exercises are subsequently those that are done with weights since they allow for progressively increased intensity. Since they can be loaded for whatever stage the lifter is in, they are far more consistent in terms of activating muscle in intermediate / advanced lifters.
Loaded ab workouts can be grouped into three separate categories:
- Machines/cables for exercises like pull-down ab crunches.
- Bodyweight exercises where weight can be safely added in, such as hanging leg raises.
- Compound lifts that engage the abs, such as deadlifts, overhead press, squat, etc.
The One Exercise to Rule Them All
Unfortunately I have not seen any study that defines one exercise over others as empirically best (though I am sure there is a ton of "junk science" from all of those "get ripped quick" ab programs). This is not to say the study doesn't exist, simply that I haven't seen it yet.
Personally I have gotten a six pack from both loaded ab exercises, and then again from compound barbell lifts after I took a long break. Both have worked well for me. Presently my focus is on heavy barbell compound lifts, but after the main lifts I do weighted cable crunches (and hyperextensions for balancing antagonists).