Well, there's no one drug that's "The Best Mass Gainer". What will work "best" for any one person will depend on various factors.
For example...do you do alot of moderate-high reps and volume? Or do you tend toward more strength training? The first scenario would suggest a test derivative like EQ, the second a nandrolone plus a DHT. Other considerations might be your current body composition, how easy is it for you to eat in a surplus, what is your planned training periodisation during the bulk, how long are you planning to bulk, what are you planning to do after the bulk, etc.