The best thing to drink when you get dehydrated is plain water. Plain water is the best drink to keep your body hydrated and especially when the whether is hot you should drink plenty of water to keep from being dehydrated.
You should avoid drinking drinks that have too much sugar such as soda, sugary tea etc during hot whether or when you're dehydrated.
You should also avoid drinking alcohol since alcohol will dehydrate you more.
Best thing for dehydration is plain water or even tea without sugar.