Water is the first logical answer to the question of fluid replacement to prevent dehydration. However, during non-workout times of the day, the research on this topic suggests that most fluids are effective at rehydrating the body, including caffeine-containing beverages.
Researchers at the University of Nebraska have shown that caffeine-containing beverages actually promote hydration and do not cause dehydration (thus dispelling the myth that caffeine-containing beverages cause dehydration). Their research showed that caffeine-containing beverages hydrate the body just as effectively as drinks that contain no caffeine. These researchers have completed other studies that show most non-alcoholic beverages will promote hydration.
This interesting research showed that subjects who drank water as their main fluid source achieved hydration levels that were similar to a group who drank any other beverages other than water during the day. Therefore, during the non-exercise hours of the day, it seems as though any liquid will hydrate your body, as long as you drink plenty of it. However, fluid consumption during exercise is a different story.
During exercise, the isotonic sports drinks (Gatorade and the like), have a definite advantage in their absorption speed. Their formulation ensures that the fluid empties from the stomach and enters the bloodstream rapidly. Therefore, during a workout, isotonic sports-type drinks are probably a better choice. For all other periods of the day, most beverages will help prevent dehydration.