a simple side plank is the easiest. The more dynamics you ad, more challenging it comes.
As the muscles of the core act in so many ways against forces in so many different directions and planes of motion, I don’t think it would be possible to have one position or motion be “the best”. And since it is relatively simple to include activation of almost all of the core/stability muscles in the performance of other resistance exercises, why not do so as often as possible and as much as the client’s fitness allows.
For research, see anything by Dr. Stuart McGill, researcher on spine mechanics, here’s his website:
discussion of “core”, Part 1 and Part 2,
http://www.youtube.com/watch?v=tr2xlwXwrWA&feature=related , http://www.youtube.com/watch?v=IyMm9oq2AgY&feature=relmfu