Ballet is definitely the most important dance style to succeed as a dance. You will not be successful without having taken ballet. This is your core form of dance. It helps you be flexible, strong, and it teaches you technique.
The most fun dance style is a hard question because this depends on who you're talking to. Personally, I would say hip hop is the most fun.
A lot of other fun forms are:
-tap
-jazz
-modern
-contemporary/lyrical