WATER
Really though, I drink a lot of water xD I started at uni for "health reasons" and now I literally can't go through a day without a couple of liters.
I think when it comes to coffee or tea it's tea, because there are too many types and I can drink them all in one day. I kinda prefer green types, because some of black teas I can't drink without sugar like Earl Grey, but then I hate the tea+sugar aftertaste.
Coffee is great in many ways and it's amazing for my low blood pressure, but I am nervous enough and sometimes it makes me more hyped than I'd like to, in a bad way x)