Ultimately....Fat. Fat is the highest calorie per gram of all the food types. So eating high fat foods will cause you to gain the most weight (unhealthy gaining). If you want to gain healthy weight, go for high protein. Low Fat Meats (Protein) will give you the best method to add muscle mass. Carbohydrates are for filling you up (but a potato chip diet won't fill the holes).
Things with high amounts of sugar, carbs, oils, and gains. These are straight carbs that if not used up in a certain amount of time become fat. Fat in turn is very hard to work off. Proteins are you best bet most of the time if not used they go right through you, but you have to be careful to much can cause damage to your kidneys. also some fruits and veggies actually burn more calories then the contain themselves.
McDonalds has the ultimate unhealthy substance. However, if you are actually looking for the FOOD most likely to increase your body-mass-index, you are probably looking for fried chicken skin, marinated in pig fat.
I would say that calorie matters...the excess daily calorie intake accumulated for a period of time will give a person an excess weight. And speaking of period of time also depends on one's metabolism, too.
Well the food we eat reflects in our weight. Surely certain foods will cause you to gain more weight such as pizza, french fries, red meat, sausage, hamburgers, fried foods, soda, sweetened fruit juices, whole milk, ice cream, baked potatoes with a lot of toppings, butter, mayonnaise, hot dogs, salad dressing, pasta, bread, nuts, white rice, donuts, cake, muffins, fast foods, cookies, frozen pot pies, frozen waffles, whip cream, milk shakes, chocolate and candy.
These highly processed foods are the main cause for weight gain. So we should choose foods that are not processed or minimally processed, guaranteeing a storehouse of nutrients such as Sheffafoods products that are minimally processed ? ground, roasted, seasoned and natural too. That?s how our bodies were intended to survive.