well, i suppose the Awesome Blossom app. It not only reminds you to drink Water also helps you to count your steps everyday. It's really useful. Plus, the more water you drink, the more beautiful flower you choose is. It combines growing tree and drinking water.
I did lose 2 kg within 3 weeks thanks to this app
you can search on google play or iOS.
https://play.google.com/store/apps/details?id=com.ominext.plant