What is the most fattening food? Trying to gain weight..?

Don't eat fattening foods, they're not good for you (duh), even if you're trying to gain weight. To gain weight the healthy way, consider the normal amount of calories you consume daily, and eat an extra 500 calories per day. This will help you gain one pound a week. You could try yogurt, fruits, vegetables, whole wheat carbs, etc., to make up those 500 calories, but don't resort to fast food or greasy stuff to gain weight fast.