even coffee, despite the caffine, hydrates you. the caffine just doesn't null out the water that's present.
its gonna vary from drink to drink. crystal light is full of fake sweetners and flavors...does that cause the body to use more water to flush that from the system, causing more dehydration than straight water??
I don't know the answer.
what I do know is that water is best, and other things should be done in sensible moderation. (e.g. the fattest teacher at my old high school was the one drinking 4 liters of diet coke a day...)