The optimal BMI seems to be 22 according to Wii Fit. It seems to be based on this on women and coronary heart diseases.
This result is based on the assumption that BMI can be accurately measured when it facts it varies over the course of the day. What if my BMI ranges between 21.7 and 22.5 over the day? P.S. It doesn't - my BMI is over the 22 cutoff which makes me wonder who made the 22 cutoff seem so great.