Raceboard Raceboard Raceboard!
It is the most accessible to a wide range of skill levels and allows for exciting racing even when the wind is too light for formula boards. Raceboard should be the primary class, which forms the base of your fleet. Some of the earlier posters brought up the excellent point that the raceboard class allows many different styles of boards to compete; almost anything with a daggerboard can be used. For example, I sail a Kona ONE, but I compete in the raceboard class because there is no one-design fleet for Kona in my area.
In my area, only the big, strong, experienced, aggressive, wealthy men race formula, whereas the raceboard class is a good mix of different types, including women, kids, and old people.
Having said that, if your fleet grows and you have many aggressive, expert windsurfers, then you could create a second class for formula windsurfing.