In a recent match it seems that Rafael Nadal is calling for a popular sports drink called Recuperation. It does sound like it - you can see the video here. In the video he looks at the drinks supply on the court and then, when he doesn't find what he's looking for, calls out what sounds like 'recuperacion' (in Spanish of course, but it's almost the same word). Of course that doesn't necessarily mean that it is his favourite drink, only that he wanted some of it at that time.
Many of the things Nadal likes best are listed on this website, though it's not his official website. His favourite drink isn't mentioned here. One thing that is known about him for sure though, is that he is involved with responsible drinking campaigns that promote safe and careful levels of alcohol consumption.