I actually don't think there is an answer to this question. The path to "effective abs" is probably getting in the habit of just using your core with everything that you do and making that the norm.
I think the only reason I feel a particular ab/core exercise is because I haven't done it before or in a while. But there is no single exercise that I can do that I feel every single time. Which is why I think the "best" ab exercise is teaching your body to recruit your core muscles all of the time.
Did I tell you guys that my physical therapist taught me that one of the easiest ways to activate your transverse abdominus is do your kegels? :D Fun, eh?
Do you, as instructors, get tired of educating people on form over and over again?