As far as possible try and stick with water for the most part. The more you allow sports and fruit drinks the more your child will get used to this taste.
Fruit drinks and sports drinks do have a lot of sugar in them so they really should be limited. Be aware of packaging- a lot of kids drinks say words like "healthy, packed with Vitamins and Minerals, only natural sugars" but what is really important is to read the back- some of these drinks with these words on the front can have over 10 teaspoons of sugar per serving- more than Coke!
If it has been a particularly hot day and your child has been really active you could try coconut water, or diluting coconut water with water to reduce some of the sugar content. Another idea which kids like is to add some fresh fruit to water- pineapple slices, berries, kiwi fruit and orange all work really well.