I've been using the Pure Protein Vanilla and Mixed berry powder. It's delicious. I'm not on a full liquid pre-op though so I mix in some fruits to get in my 4oz for my morning and evening meal.
They also have pre-mixed shakes in a can they have much more flavors or pre-mixed bottles. I didn't like the ones in a bottle. It had an after taste.
I heard unjury was good too and that they have an unflavored one for post-op so you can get your protein in different ways than just shakes.