Even if it's 100% juice, it's still going to have lots of sugar (naturally occuring in fruit), which means water is probably better for ya. It doesn't hurt to have a couple glasses of juice a day, in fact that can be pretty good for you because of the things that are in certain fruit juices (grape juice has lots of flavonoids (sp?)), but you should drink pure water more than anything else.
Tea can be good for you too, I believe it is another source of flavonoids (flavonoids are what make wine good for you, except alcohol causes the brain to shrink...juices and tea don't do that), and there are plenty of teas that do not have caffeine.