There's no such thing as a "mass gainer." it's just calories. Protein and carbs have 4 calories per gram and fat has 9 calories per gram. So technically, what builds the most mass is the most fat. Of course, high sugar protein drinks are an easy way to get more calories. That's fine as a supplmeent when you're desperate to bulk, but I wouldn't depend on them.
Blend whole milk 4-6 eggs, peanut butter and several scoops of whey. There’s your mass gainer with not a lot of junk fillers out in assuming it’s a cleaner protein powder