Pineapple, Grape and Apple juice are the main ones. Their are a few others but I would have to look them up. Grape masks the taste the best. I haven't tried Pineapple because I noticed it had a higher vitamin C content. Only organic juice should be used as it has nothing added. Frozen concentrate will also work and it is best to make it a little stronger (less water) to cover the taste.
I think the reason Humbles Protocol calls for 15 drops is because the juice weakens the dose a bit and buffers the stomach.