Squats and Milk!!!
Though I don't pound the gallon of milk a day lol. But been trying this out since plateauing on squats.
But I agree with the above. If you're looking for mass, squats and deadlifts are king. If you can't do barbell squats, do the 100 squat challenge instead. I would personally say pull-ups are essential for building mass on the back. Don't even need to throw them into your regular gym routine. Get a pull-up bar, throw them up on a door frame, and do a few sets every couple of days. Once it gets easier, throw on a backpack, throw some heavy stuff into it and pull up with that.