By the statistic, underweight and obese have significant increases in mortality. Whilst the overweight (by the BMI) have an overall decrease in mortality compared to the normal weight.

Obesity puts too much pressure on your joints negating the lubricants. But underweight reduces the lubrication in your joints. Overweight seems to be the ideal, as it has for thousands of years.

Obesity is correlated with higher LDL cholesterol, and underweight is correlated with low levels of cholesterol overall, which can reduce vitamin D production and in turn increase your risk to virtually every disease and cancer out there.

For my body fat percentage, I sit in the low 20's, which puts me into the overweight category. Which, incidentally means I'm likely at a reduced risk of stress on my organs and joints, as I've seen little to no evidence of a correlation between muscle mass and arthritis or other weight-related disease.