I think Carnation Instant Breakfast is really good and tasty...loaded with nutrients too. They have a variety of flavor packets (powder). There's always Ensure too, but it's pricey and taste like medicine.
Carnation Instant Breakfast is the most inexpensive way to go. Put a glass of milk, the mix, maybe a banana (or other fruit) in the blender, and you're good to go. My mother's doctor gave me this advice when my mother lost her appetite while terminal with cancer.
Oh Ensure is just awful. There were some leftover cans after my dad passed away. I drank half of one and tossed the rest. I remember Carnation Instant Breakfast. It was pretty tasty. Thanks for the tip.
Avoid Carnation Instant Breakfast. One of the main ingredients is Maltodextrin (basically hydrolyzed cornstarch). This is a high glycemic starch and will wang your blood sugar around and promote fat gain. Verra bad.
Check out MRP's (Meal Replacement Powders) like Prolab's Lean Mass Matrix. It is more expensive but much, much healthier.
I recommend the Cinnamon Oatmeal. They also make a low carb MRP (or you could just use less powder), but I don't think it's quite as yummy.