Well, my absolute favorite has to be root beer but I usually try to stick to water and milk because I have a problem with sweets. Used to drink half a case of root beer a day and a ton of chips and swiss rolls and other sweets. Now I only buy them every once in a while because I go nuts (btw drank over a liter of root beer today).
Back in my younger days it was hands down milk, I was known around as the Milk Knight, Hero of Milk, Milk Man, you know titles that really showed off my love for the stuff. Anyways, after a few years of downing the stuff daily as if i was in a desert and found an oasis, I developed the worst possible thing for any lover of dairy as myself... Lactose Intolerance. Lucky it wasn't so bad to cause me to feel sick when i drink it, it just makes everyone want to avoid me for a few hours... Now that i can only drink a bit a day, my favorite drink has changed. Apple Juice is nectar of the gods now. There is a reason it's gold.