What is the quickest, yet healthiest way to gain weight?

Not the same: The only healthy way to gain weight is to undertake a fitness-focused lifestyle with an emphasis on both resistance and aerobic training. A man wants lean muscle and needs no body fat whatever -- most cultures now strongly prefer this. A woman needs only a modest amount of bodyfat and looks & feels best if physically fit. Both are also the healthy choices. Assuming you're healthy, no GI disease.