All of the above at one time or another. Last 200 I did which was about 235km I drank mostly water, might have bought a flavoured drink and added to my bidon. In the past I have used Isostar but don't really want to carry so much for the whole ride. Usually start out with it then switch to water as I go round, or buy a Oasis or similar if I need some taste. But wait all of that is for rehydration not energy. There are no calories in water. Most of my energy replacement comes from solid food, ordinary food that normal people eat that I can get on the road, so I don't have to carry it.
200km is not such an ordeal, one can usually get round on just about anything including body fat. However if one is intending to go further one needs to use the 200s for training and preparation so one doesn't want to have to use a different strategy for 300, 400, 600, 1200, 1400 km rides.