Protein coffee. Not saying you have to make it this way, but, I've found this to be the best way.
Protein powder flavors I'd recommend. Vanilla, Mocha, or Chocolate.
Coffee: I'd recommend ours, obviously, shameless plug. But, I'd recommend it be cold. When you have hot coffee I find that protein powders tend to clump up if you don't have a blender or something to quickly whisk the powder in.
Also, add in some milk, not a lot but not too little either. You can also use coconut milk...that's not too bad either actually.
The frothier you make it the better!