If you've read my blog in the past, you already know my opinion on core and ab training exercises: the majority should be stability-not mobility-based. Many people-in their quest to get "rock hard abs"-fail to understand repetitive flexion, extension and rotation of the spine is not desired, and may actually end up causing back pain and problems.
The core-including the desired "6 pack" ab muscles-is in need of more stability. With this said, one of the best stability based core training exercises is the Pallof Press (named after the Physical Therapist John Pallof who came up with the exercise):
You do not need to use a lot of resistance for this exercise. If you have to lean to one side, are getting rotated inward, of can't keep your elbows completely straight, you need to lower the resistance. I like to do this exercise for timed holds: try 2 sets of 8 reps, holding each rep in the extended position for 8 seconds, twice weekly.