Thirty years ago, scientists thought water was the best fluid replacement.  While that's still largely correct, sports beverages are sometimes superior because they provide water, energy, and electrolytes.  Milk might be even better, because it sustains water balance longer and provides a protein source that can promote tissue adaptation and repair after exercise.

British researchers found that low-fat milk restored and sustained body water levels in dehydrated people better than water or a sports drink.  Because of urinary fluid losses, subjects were again dehydrated one hour after consuming milk or milk plus sodium.

A review of literature by British researcher Suzane Leser concluded that consuming proteins in a post-exercise rehydration beverage increases absorption of electrolytes and boosts levels of plasma proteins, which promote long term hydration after exercise.  (Nutrition Bulletin, 36: 224-234, 2011)