I highly recommend consuming a balanced diet, but a high-calorie one.
Don't just eat a lot of foods that are high-fat just because they are energy-dense...I did that and it backfired to an incredible extent. The high-fat weight-gain 'diet' made me feel sick to my stomach at times..it was simply very rich, at times greasy, and not that healthy for my body. The high-fat foods I was consuming were olive oil, canola mayo, cheese, whole milk dairy products, challah bread, nut butters, avocados, nuts, and seeds.
After three months on this high-fat-weight-gain-diet, I began to CRAVE FAT..constantly. The more I ate, the more I would crave. After realizing that I was consuming upwards of 70% of my daily calories from fat, I knew it had to stop..I was killing myself, even though the fats were basically "healthy" ones. I lowered my fat intake and after some withdrawals (intense cravings for fatty foods and the texture/taste/mouth-feel of fat), I felt so much better physically and mentally.
I replaced the high-fat intake with liquids. Soy milk, yogurt drinks, 100% juice, smoothies, Ensure, etc. I still eat high-fat foods, just not in excessive amounts like I was before.
I now have a pretty much balanced diet of about 30% calories from fat, 10% calories from protein, and 60% calories from carbs. Most of my carbs are those that are complex (whole grains), but some are simple (sugar, juice, honey, jams, flavored yogurts, etc).
Once I reach a healthy weight, I will simply lower or completely stop (whichever is needed) my intake of caloric liquids and extra additions (such as honey, jams, condiments, spreads, etc).
However, everyone is different. You might be fine with a high-fat diet, but I don't recommend it. Ensure does taste kind of nasty, so you probably want to avoid that as long as you aren't dangerously underweight. If you add 1 tbsp of nut butter and 1 cup of 100% juice to your daily intake (not a lot of food..very easy to do), that would add about 250 healthy calories/7 grams of healthy fat/ 10 grams of protein/30 carbs daily. That would translate to about a 0.5 lb gain per week..which is 2 lbs per month..or about 24 lbs a year.