What is the most effective Colon Cleanser over the counter?

I’m in a contest with my husband. He has always been overweight, and I recently became overweight with the birth of our third child. (all born within a 3-year period.) The contest ends on January 1st (and we hope that the weight lost by then will be motivating enough to keep us going.) and I heard that colon cleaning can be a great kickstart to any diet.

We’re doing this all natural with a change of diet and exercise. No pills involved except for Colon cleansing. So I’m wondering what I can buy at GNC or some other drug or health food store.