To not turn this into a health or personal opinion debate, let's define "healthiest" as having the least sugar and most protein. Tomato juice seems like a solid option. Any others?
I see Minute Maid Orange Juice, Canada Dry, (Diet) Sierra Mist, Sprite, but there are also some mysterious bottles and cans that I haven't had a chance to examine.
Water, water and water. Drinking something is probably a good idea and with water you wouldn't get any sugar, which even seemingly healthy options like milk or fruit juice have in excess (thus optimizing your first criteria). For the rest, you should get food anyway (not necessarily during the flight, if it's short) so there is no reason to compromise on the sugar to get “more protein”.
